I'm going to agree with Blizzard's policy but for different reasons than Rohan's and I'll actually have two separate arguments. The first is more general and applies to social bonds and opportunity cost, the second is directed at progression raiding.
On Social Bonds
This may shock people, but Blizzard charges a monthly subscription. Thus, they want you to stay subscribed.
It's possible that you've considered quitting raiding and/or WoW, for whatever reason. But when you did, it's likely you felt like this:
One of the main points of MMOs is social bonds, building relationships with people in a persistent world. Those relationships are what generally keep people playing even after the available content is consumed. Doing a dungeon for the 50th time probably isn't fun. Doing a dungeon for the 50th time with friends might be.
So how does this apply to cross-realm raiding? Well, let's say you're considering quitting but you're part of a raiding team. In which of these two scenarios are you more likely to quit?
1, to fill your spot and allow the group to keep raiding, someone just needs to have a friend somewhere on some server some character that's able to fulfill your role.
2, to fill your spot and allow the group to keep raiding, your guild has to recruit someone, most likely cross server. This person has to go through an application process and be vetted and then pay a transfer fee, at which point your guild can evaluate them in a raid environment and see if they're an adequate replacement.
In terms of social bonds, it's a lot easier on you in terms of quitting in the first option. You may feel guilty for making them do that work and you know you're likely introducing a complete stranger into the group of friends.
But there's more.
The man in the above image is discussing the idea of "opportunity cost." In short, opportunity cost means that whenever you do something, you're giving up doing something else you could have done instead. To summarize the summary...
"Time is money, friend!"
Let's say you have a mage in your guild. He's not performing quite as well as you'd like, and you're considering replacing him as a result.
Currently, to replace him you have to advertise, go through applications, pick someone, and have them transfer over with the knowledge they may fail their trial and thus have wasted their money. That's probably quite a bit of effort and time invested. In short, it puts up a barrier that means you're less likely to replace someone on a whim or for a very minor improvement.
"Good news, everyone! We spent a week of slaving over applications to replace our old hunter with a new one that's 1% better! Totally worth it!"
On the flip side, if you can replace him with someone who's a friend of anyone cross-realm, that lowers the barrier to replacement and means you're *MORE* likely to replace people because the opportunity cost is lower.
Note that this also can work favor of new applicants. Let's say the old hunter actually had to quit raiding for whatever reason. You get a new hunter, but he's 1% worse than what you want. Is it worth going through the hassle of recruitment again to try to find a better hunter? For the vast majority of guilds, the answer is no.
In short, putting up barriers (by not allowing cross-realm raiding on the current tier) helps maintain guild bonds because the opportunity cost of recruiting someone new (or leaving for a new guild) is much higher and so it happens less. It makes moving between groups less fluid and preserves more relationships, which keeps more subscribers, which makes Blizzard happy (and is good for the game).
On Progression Raiding
All that said, allowing cross-realm raiding also causes issues in the progression arena on at least two fronts...
The first front is more obvious: realm firsts and realm rankings.
Note: that's not my achievement.
Allowing cross-realm raiding for the current tier muddles these waters. If Guild A borrows someone from Guild B cross server for a server first kill, is that considered a realm first? Or let's make the example really horrid...Group A uses nine of their people in a cross realm group with Guild B on Guild B's server to get the realm first on that server.
Or let's say realm firsts aren't even involved. Let's say my guild is 16/16H in a month or two and we don't need one or two of our raiders for, say, Heart of Fear. So they go help some friends on another server who are working on normals or early heroics in Heart of Fear. You might imagine that the competition on that server might not be thrilled about this.
Are realm firsts and realm rankings outdated and archaic? Perhaps, but a lot of people still enjoy it. It's a lot more fun to say "We're 8th on the realm" instead of "We're 8,589th in the world." Note:
those numbers are taken from an actual realm.
Is all of the above possible within the same server? Yes, but trying to pull off the above within one server is a lot harder than when you have access to every server.
The second front is somewhat tied to the first front: maintaining an appropriate roster.
One of the challenges of progression raiding is striking a balance with your roster. You want enough people to handle all roles and to cover absences, but you don't want so many raiders that people become unhappy due to sitting and loot gets diluted (a 10 man roster gets 50% more loot per person than a 15 man roster, to use some extremes). If you try to raid with exactly 10 people and someone has to miss a night, you're in trouble, and that's the penalty you pay for trying to play it fast and loose.
Allowing cross-realm raiding for the current tier encourages people to use smaller rosters because finding a replacement is a lot easier. You just need one raider who knows some person who can fill in the spot for the missing person. As a result, it places less emphasis on the guild as a self-contained unit, which weakens social bonds, and we've already discussed how Blizzard doesn't want that.
So there you have a few reasons against cross-realm raiding for the current tier. Allowing cross-realm raiding for the current tier will result in...
1, more people quitting raiding and/or WoW due to ease of replacement
2, more people being replaced in guilds since the barrier of replacing is lowered
3, more people leaving guilds because the barrier of trialing for new guilds is lowered
4, weird (and detrimental) effects on realm firsts and realm rankings
5, less emphasis on raid groups as self-contained units which weakens social bonds
When you're sitting there and missing one person for a raid, I'm sure seeing a bunch of friends online cross realm seems to taunt you. And individually, letting you bring that one person doesn't cause an issue. But WoW players have a tendency of taking things to extremes.
To draw a parallel, taking a friend to LFR with you wasn't really a big deal. Allowing that, however, resulted in guilds doing 10 runs a week using 3 new characters and 22 "saved" characters to get people tons of loot. If you open the door, people will abuse it.
Perhaps you're still thinking "So what? Social upheaval is fine, people will eventually sort themselves out, cry havoc and let loose the dogs of war!"
Whenever social upheaval occurs in WoW, guilds tend to dissolve and people tend to quit. So how much stability are you willing to give up for fluidity? How much are you willing to sacrifice on the altar of cross-realm raiding?
That's an answer you'll have to decide for yourself.