Coriel on Blessing of Kings mentions that the real problem isn't simply the low population, it's also player retention. Hardcore gamers leave the low population realms to go where they can achieve their goals, namely raiding end-game content (I would also add high -level PvP to this.) Simply having more bodies to a server won't keep that player there. Coriel's analogy of server buckets with holes is a valid one. The only thing that keeps a hardcore player on a server is a guild (or arena team) that can get them where they need to go. The guild is the plug that keeps the leaky bucket full.
While some might look at this as elitist, consider for a moment the notion of server pride. We love our realms, love to show off the achievements of the players we play with. Even if we aren't the ones raiding Mt. Hyjal or the Black Temple, there is still a certain level of excitement on a realm that has guilds that are pushing through the progression. I think this applies to the entire realm, regardless of which faction you happen to play on. It's that energy and excitement that make the difference between thriving realms and dying ones. More active hardcore players means more gold flowing for enchants, for consumables, and the economy grows.
I definitely see Coriel's point, but I wonder then, what is the ultimate solution? Merging a low population realm with a medium one, a server with a few healthy guilds? That might work well, as long as the influx of people doesn't begin to swing the pendulum the other direction, and then the problems of overpopulation creep up. Add to this the challenge of keeping a proper faction ratio on a server, and it all becomes a sticky mess.
Still, I think I may have hit on something here. It certainly would make it simpler than recruiting guilds specifically to switch servers in order to help out a struggling group of players.