Well, WvW isn't intended to be balanced, so if it does have an impact (which I find unlikely) it won't be a big deal. I can see some guilds having a "Home server only" policy, so you can only join if you're from that server, in hopes of preventing that. In general, though, cross-realm servers will mostly matter to people who want to join a guild with friends but didn't create characters on the same home server. Of course, you might join a cross-realm server if you meet someone in a dungeon/sPvP that you enjoyed playing with.