Without electoral reform, there's no functional reason to. The lesser evil of the current system is Democrats. If you want to enact change, push support for the progressive wing of the party and push for electoral reform.
I think I'd agree with you more if I believed "pushing for XYZ" was a valid strategy for implementing structures opening the door for progress in a system dominated by those who benefit from the status quo. Without that belief, I turn to other historically succesful methods of gaining change, like millitant labor organizing and revolution.