It depends. I have many themes for every one I release, and only a handful of those get to genuinely useable points. For me, making a theme that is to be released from the beginning is the only way to keep on the goal, and not get sidetracked by, "Oh, let me if this could work...".

However, when I want to see if something can work, a new theme for it, and a few other things, is about the only way to work it. These generally don't make it anywhere past filling up my themes directory. OTOH, the first class of themes are actually meant to be used in some form, where the second are made purely for technical/creative insight.