Well, I'm here to outline the future where cross-realm raiding goes badly.
If you look at BBB's post, there's an underlying, unstated assumption. He assumes that if you need a raider for that last spot, you would pull from your guild, then pull from your cross-realm friends list, and lastly pull from trade chat on your server. That there is a priority list that goes:
Now suppose that assumption doesn't hold. Suppose with the advent of cross-realm raiding, the priority list becomes:
I.e. you pull from your friends list before you pull from your guild.
This would do immeasurable damage to WoW in the long run. If you're someone new to end game, how would you break into raiding? Right now, the path is you apply to a guild, and the guild takes you on runs. The application and acceptance does not require an existing friendship. Both sides understand that they are taking a chance on strangers.
A lot of people don't like guild apps because they are impersonal, but that distance sometimes makes things easier. The guild app process provides a path where strangers can become friends through raiding together. Every new app is a stranger to the guild as a whole, not yet a friend.
In contrast, breaking into someone's circle of friends will be very hard. They'll be raiding with their friends, so you can't play with them. The barrier to entry for raiding could become much larger, and the barrier between strata of raiders could become even higher. If you're a Royalty raider, is it better to raid with another Royalty raider from a different server, or take an effective stranger from your current server?
Now, you could say that this future isn't likely, that no one will prioritize Friends over Guild. (Though, wouldn't many people say you should prioritize Friends over Guildies?) But one never knows how people will react. I would have never predicted everyone rolling Need in LFR, and subgroups funneling loot to their own members. And obviously, neither did Blizzard. But that happened.
As well, prioritizing Friends over Guild does provide two distinct advantages. First, you get to play with friends, as opposed to people who are still strangers. Second, your pool of friends is often higher quality (and a known quality) than new guildies. If you're friends with someone in Paragon, bringing her alt is more likely to lead to success than taking your latest guild recruit. And as we've seen time and again, WoW players always take the short-term success, even if that leads to issues in the long run.
So that's the case against cross-realm raiding in the current tier. If it goes badly, it has the potential to seriously damage the method through which people get introduced to serious raiding, as well as one of the major methods by which strangers become friends in the game. It has the potential to see the raiding stratas become very tight cliques, which become harder and harder to break into.
Raiding needs new blood to keep going, and anything that has the potential to damage or block that intake should be looked at with great trepidation.