In PvE groups and instance runs, players are arranged into parties. In a raid group, where there are multiple parties, there's usually a healer in each and we stick to healing our own parties. We move forward as a group by necessity - because a single player would be highly unlikely to stand on their own against the monsters we're fighting. I watch my party's health bars, and I heal them when they get low. Easy enough, right?
But in a PvP situation...
...individual players are quite capable of taking out opponents and capturing flags in Warsong Gulch or Arathi Basin. This frequently leads to players running off in every direction (though less so in smaller battleground of WSG and more so in larger AV), and out of range of my heals. So the usual tactic of watching my party interface and casting heals based on what it shows me stops working - i may be within healing range of half the party while the other half is on the opposite side of the map. Even if I have health bars for the entire raid displayed, there's nothing to judge who is and isn't within range of my heals, and attempting to heal based on those will simply give me endless out of range error messages. The next option is to simply watch the screen for players nearby, click on them, check their health, and heal them if needed. But now instead of scanning several players' health and casting heals on the one who most needs healing, I'm throwing heals around randomly, on whomever I can see and target first, which doesn't always coincide with the player who most needs it.
Of course a well-organized group that works as a team will never have such difficulties as I've described - but well-organized groups aren't always an option for players without membership in large PvP guilds. (Myself, I'm mostly a PvE player, and in a PvE-focused guild - which doesn't have enough players interested in PvP to front fully organized groups.) And while semi-organized pick-up groups may work out fine for DPS classes, while healing classes fall apart.
I'm hopeful that we'll see some changes in patch 1.12 (which is rumored to include a PvP revamp), regardless of what they may be, that make it easier for players to heal in PvP combat. But that could be a long time coming, and in the mean-time there's nothing to do besides cope with the situation as it is. Perhaps there is some tactic I've yet to realize, and despite my frustration, I'm keen to keep trying to figure it out.