100% Yes you can and should use ad-hoc testers. I think the real problem isn't in finding bugs, but in changing direction/adding value which isn't always surfaced in QA.

The hard part is that if we assume the "live fast/die young" model it becomes increasingly difficult to change/grow the further we get from launch since we are falling further below the more recent apps, below more recent items on blog, social bookmarking, and pr sites, and our buzz is wearing off. Unless we can re-invigorate the buzz the work put in to updating the app might be better spent creating version 2 (w/ it's own buzz and launch) six months down the road.