But at the same time I never would have bought the app if
  I didn't use "free" for several weeks and got the idea it 
  was useful.

I second this. Having experienced the free app and having bought the full app as a result has also resulted in two other people directly purchasing the app. There's a network effect that starts at the free app. The free app may not be necessary anymore, but I doubt it was never necessary in the first place. I think that would have flattened the sales curve quite a bit.