Ancestral skills permit you to select between two versions of three available skills in the trees. They are very useful in their own way. It doesn't give you anything else. For the majority of the trees, you should be using at least one of the skills provided. From the three classes I rotate, I use ancestral skills from Vitalism, Auramancy, Defense, and Songcraft. The list is limited, but you should still be using them if they're recommended by those who play the class(es) you do. What else would you be looking/wanting from Ancestral skills?
I think that you should be allowed to assign different ancestral skill profiles to your current saved skill load-outs. I was really disappointed to see that's not the case, even though it is with Abyssal. For example, being able to switch between standard Tigerstrike and the one that only attacks one opponent would be extremely useful for building a specific PVP mode. This sort of change will encourage more play in my opinion, so it's good for everyone. Anything that CAN be made more convenient to use, always should.