The longer you perform material, the more it becomes you, and the more you can focus on the details of performing and your relationship (and communication) with your audiences.
If you are a conscientious performer, over the lifetime of a performance piece, you will likely consciously work on improving it, but it will also gradually evolve on its own until it reaches a “finished” status, where the hard and fast dramatic elements (plot, script, movement, character, etc.) fuse with a sensitivity for “the moment” (context, time, locale, particular audience) to form a unique performance. This is usually only achieved after many, many performances.
It is this fluidity—this sensitivity to “the moment” coupled with a flexibility that comes with confidence of the material, that allows you to “loosen your grip” and ride the piece reacting to the vagaries of each performance situation. In this way, extended knowledge of the material, frees you up to grow as a performer.
While I think it is always good to be broadening your abilities as a performer (expanding your toolbox so to speak), there is much to be said for performing that material which reflects you the most. This is most likely the the material which you have performed the longest, that you are the most comfortable with and that will most likely get the strongest reactions.
Our audiences deserve our finest performances, therefore they deserve our most polished material.