Certainly each of us uses them with different meanings, and like the realities they attempt to describe, they are in constant flux. However, I'm not convinced that because they are incomplete or in the process of transformation, that they ought to be discarded entirely.
Many people feel a sense of power and self knowledge in naming themselves.
This language choice is in contrast to the more governmental term, Hispanic. Consider the role of identity in U.S. civil rights struggles. And it can also be a factor in poltical mobilization. I will no longer be made to feel ashamed of existing. If I reject identity labels in hand, other people will still continue their attempts to reconstruct me, and I will be left with little way to articulate who I am in relation to those around me.