There's no question that when spawning or quest objectives are not handled properly on our side, the tap system can create negative and anti-social experiences, wherein seeing players of your own faction nearby becomes a nuisance. We very much want to limit and rectify those situations. The most helpful thing you can do in that regard during beta is to bring to our attention specific quests or areas in which you felt competition for spawns was overly detrimental to your experience. We have a number of ways of fixing those problems, ranging from simply adding additional spawns, to dynamic spawn thresholds that ramp up density as player density increases, to making specific targets open tap.
The main reason we don't embrace a fully open-tap world is that we feel that those mechanics are asocial. To be fair, that is certainly better than antisocial - no question there, and antisocial experiences usually reflect spawning and mechanics that we need to adjust. However, while a world in which everyone runs around damaging things a few times (or however much is needed to qualify for credit) may be one in which you don't feel bad about other players being around, at some point it also makes those players nearly indistinguishable from NPCs or bots with decent AI. You don't need to talk, or ask if someone has room in their group or would like to join yours. You just attack a few times, and then move on.
On the other hand, mob tagging rules inherently reward and encourage social gameplay. Even in solo areas like daily quest hubs in Mists, we'd commonly see transient pickup groups form for the sake of efficiency, and stick together through that hub or maybe even another ("hey, anyone up for Klaxxi after this?"). But once again, it's incumbent upon us to make sure that we avoid situations where that is outweighed by negatives like competing for underspawned quest targets or objects.
Philosophically, for a while now, we've made sure that any time multiple players are sent to kill a single specific target that has a respawn timer (be it named quest boss, world boss, or an event like the Battlefield: Barrens commanders in patch 5.3), the mob is open to credit for all. We're certainly open to extending that treatment where it makes sense, and where it serves to improve the overall experience. But I wouldn't expect a wholesale overhaul of our tap mechanics in the near future.