Your argument begs the question. It is the same as saying: "since evolution actually happened, these mutations must have conferred some evolutionary advantage".
The fact that we are not aware of any evolutionary advantage conferred by a change yields no information in either direction about the validity of the evolutionary theory.
http://smile.amazon.com/Why-Evolution-True-Jerry-Coyne/dp/01...
It summarizes the strongest evidence for evolution in several categories, including the fossil record; the evolution of various features like feathers, eyes, and wings; sings of path dependence that can only be explained by evolution, like the recurrent laryngeal nerve[0]; etc.
[0] https://en.wikipedia.org/wiki/Recurrent_laryngeal_nerve#Evid...