1. Not every class needs to get a new spec, this is a misconception and frankly wishfull thinking. Forcing one for each will just lead to bad design.
2. A new spec gives something interesting to existing characters instead of forcing another reroll, which usually over 90% of people won't do.
3. A new spec can flesh out aspects of classes that have yet to be properly explored.
4. If Blizzard has any sense they could even add a quest and make it unlockable and therefore content.
The problem is Blizzard has reached the limit for level of complexity that they are comfortable with for classes - or at least that they think we can handle. Because of that we are in a situation that prevents us form advancing and everything we get is shittastic rental throwaway systems and abilities. 4th specs in that regard at least provide a new parallel system within a class to get new stuff.
While I would ideally like to keep building upon existing classes and get new permanent things added to classes, that simply isn't going to happen anymore if we are real about it. But it's true that Blizzard is already incabable of addressing glaring issues with some specs as it is. Adding more will not make this better.
- - - Updated - - -
I disagree, people often picked those because that is what they wanted to do. If you can find a way to dps in another way then that is absolutely fine, not everything needs to become worse by hybridization. That people in general tend to be more interested in DPSing already shows that this would still be a viable approach.