I don't think we'll be able to switch specs at all, will he? From everything i've heard, it sounds like the way we spec is how we'll have to play, but that we can mix and match.
Example: Stealth healer, Mage-tank, Illusion ranger, etc etc.
We don't even know if there will be "specs" or not.
What we do know is that classes in ESO will be rather opened ended. Meaning picking a class simply determines which class specific abilities we have. Beyond that though we can use and skill up in any weapon or armor in the game. We also know that there are abilities tied to weapons so changing a weapon will change/add some abilities on our action bars.
It looks like so far from what we know that the combination of what class we picked initial and what weapons armor we use determines how we play and what roles we fullfil, and not some skill tree or spec we spend points in like other MMO's. We might have perks and such like the single player games but those will likely simply enhance our roles not determine them.