Here is what I see as the worst case scenario. In ranked, some team compositions will have 8 fixed players, meaning each player will play one build. Other compositions will have maybe 6 fixed players, and 2 players that can change as needed to accommodate different teams or warzones. I think this creates some competitive imbalance.
This unfairly creates a stronger need for certain classes solely based on their ability to change specs. The new bar for playing at the highest level becomes farming 2 sets of gear, which is unpleasant. I think if full dual speccing, or spec profiles were added to the game, the illusion that this wouldn't be a problem would be lifted.
With that said, this is a legit tactic right now, have fun.
I've played at least 25 ranked games since the introduction of free respecs for subscribers (likely more) and from what I've seen it's largely been a non-issue. Both sides tend to have players able to switch to tank/heals for the defense phase of Voidstar and usually both sides have those players do just that. As such, the potential advantage free respecs might give to one side has, it least from my experience, largely been negated because both sides are taking advantage of it.
Moreover, because you aren't certain which WZ you will get, the usefulness of pure DPS classes is still there. It's generally not wise to take, for example, 4 Sages/Sorcs into a ranked match just because they can DPS and heal, as you might run across a team full of lolsmashers who would likely make short work of such a team. I played four ranked matches yesterday and in each there were still Sentinels/Marauders, Snipers/Gunslingers, etc.