Part of the strategy of these warzones is in adapting the specs that people play to the particular map they are on. I understand the unhappiness some might have with it, but in the end I do not believe that it is a majority that feel this way, and, more importantly, to make this change would only further shallow out what is already becoming to dumbed down a PvP experience with some of the changes that have been and are being made.
Simply put, there have already been enough concessions for those who want a simpler/easier PvP experience. This one needs to stay in for those who like a little bit more depth to things.
On top of this, the change would only further limit the kinds of classes you are going to see played in PvP and go against the entire spirit of this expansion's PvP approach, which is to make every class viable. So far, its going well, though a few tweaks are needed here and there. However, if there is no respeccing allowed in warzones then certain classes/specs will never be played in PvP because they are just too weak on certain maps. Allowing respeccing encourages people to play whatever class and spec they like, and if they wind up in a map where their particular spec is a liability, to switch to a better one for that map.
Actually, by allowing respec'ng in warzones you penalize the pure dps classes because they can't switch to hardcore defense mode. As a result, you really don't want more than 2 total of the pure dps classes if you want to have any kind of versatility in respec'ng, and probably not even that.