How is that at all different than now. The only difference is rather than going to town and changing spec you do it on the fly. All it saves is time, nothing more. I already have access to my entire toolkit simply by going to org and spending money and time to change all of it.
The ONLY thing this would prevent is being able to handle any situation without having to leave the area. For example if you're a lock with affliction/destro specs and there is a mob you can't kill except as demo you'd be able to swap to it without missing your shot at that rare because someone will have likely killed it by the time you return. That's the only thing this would stop which to me isn't really a big deal. Whoever gets their first should get the shot, not whoever gets their first and happens to have the right spec at the time.
This whole "makes specs meaningless" is complete BS. Nobody gives a crap about your stupid spec, it's already meaningless with the simple as hell talent system and the ability to change glyphs/talents on the fly with 20s. The only thing it does is remove money from the economy for people who change specs often (this probably doesn't happen anymore) and wastes people's time (having to go to town).