Before I start, let's get one thing straight: the building community fallacy needs to go away. At the very least, some clarification is required.
Servers/Realms/Shards don't exist to breed communities. This is, at best, a by product. The real reason behind their presence is to bridge technological gaps and hardware bottlenecks. Any person with an IT background understands that easily. I won't lecture you on Sharding, but you know what I'm talking about. Building unified/monolithic systems is very expensive and hard to maintain (in house you probably have your Auth/Subscription systems which represent that issue).
As it is right now, players are gated within their own servers. They are forced to play with or against a particular set of players. This also means they are penalized whenever they can't play during their server peak hours, because the pool from which they can pick peers to party with/fight against is restricted, sometimes, to the extreme.
For some, this is good. They have their sense of a virtual community fulfilled. Maybe they seek recognition within that gated community. Maybe they are afraid, or simply don't want to deal with, of what the anonymity (inherent of any MMO) within the anonymity (partying with players from other servers) could bring. That's understandable. I would classify these players as leaning towards Role Players, not that they necessarily do RP, just for the sake of labeling them.
On the other hand, you have players that don't care about that. Players that have had different experiences with similar cross-server functionality. People that want to experience what's called in other game communities "co-op". They want to band together, smash their way thru a Flash Point (or whatever name is there for instanciate content) and, whenever they are done, go about their ways. They don't want to be forced to play during specific schedules to increase their chances of experiencing co-op/group content. They don't want to engage in social interactions to enjoy the game. They just want to play it. Let's calls this group the "CO-OP" players, again, for the sake of labeling.
What you have out of this, and I have seen this scenario in other games as well (namely, Rift) is a clash between these 2 groups.
The RP side fears that cross-server grouping will let them without peers to play, or even worse, force them to party with players from other realms. For these reasons, they are vocal against such a feature.
The co-op group feels penalized for their desire to just play a cooperative game. They feel forced to play on specific schedules, to spend time trying to form groups to enjoy the content meant for small parties, to engage in social exchanges that aren't significant for them. On top of that, players on low populated servers are even more penalized, because it's even harder for them to experience group content.
How do you solve that problem?
1st You need server transfer technology. That's something inevitable in any MMO that lives long enough.
2nd Create servers that are gated. If you play there, you only play with/against people of that server. You will never interact with players from other realms.
3rd Grant players a free move to these servers. Balance your gated servers according to interest.
4th Behind the scenes, implement a cross-server grouping mechanism for both PVP and PVE content. Such a functionality wouldn't be available on gated servers, for obvious reasons.
5th Make sure the grouping mechanism has a opt-out option for people that don't want to move servers, but also don't want to party with players from other servers.
This way, you are not forcing people to agree with each other, nor you are coercing a particular play style upon your entire community. Those who seek social recognition within a specific group, can do so. Those that want to play a co-op game, can do so, whenever they feel, for as long as they want.
In the end you have a happier and healthier player base that will keep playing your game for a long while.