I tend to like non-idiomatic improv most in solo performance - the solo work of Derek Bailey and Evan Parker, in particular, is great. In a group context, the desire to avoid cliche often seems to result in people playing at cross purposes or just plain not listening to each other.
Also, it's incredible how cliche-ridden and conformist improv can actually be. The last improv show I went to featured a lot of people blowing into their wind instruments with the mouth-pieces removed, resulting in a uniform puffing noise. It was totally ridiculous but they were all doing it because I guess that's what the cool thing was in improv at that moment.