Respec costs aren't that big of a deal unless you're doing it multiple times per week.

If you respec once a week and then spec back, your gold for that week is 100g. BFD. You'll spend far more enchanting/gemming a second set of gear. Even that isn't much though - most enchanting mats can be gotten via DEing greens (not an enchanter? Make some friends....). Gems? Green gems are fine for an offset that's changing a lot. Blue gems are a bit pricey, but are only going to be used in gear that's epic, so it's more of a one time cost.

This feature would have made more sense in TBC actually, when a lot of tank classes couldn't do reasonable dps as tank spec and could have used a low cost option to switch to dps. IN fact, I think this will be the nicest thing about dual specs if they aren't limited to 80s - you can level as a dps spec then respec on the fly and pop into an instance to tank or heal.

The issue for pure dps classes is that, aside from alternative specs for PvE and PvP, the benefit is minimal to them.