My reasons for wanting a flip shell
Disclaimer: I've ordered but still haven't received my phone or flip shell yet...
... but I opted for the $30USD flip shell because:
I wanted a way of covering the screen when it's in my pocket but I didn't want a screen protector.
I like the form factor of the phone and I want to be able to show it off.
It's gotten decent to good reviews from everything I've seen and read online.
The fact that it turns on/off by just opening/closing the shell is a nice novelty.
NOTE: I wasn't terribly happy with the screen protector on my last android phone (HTC Incredible S). It worked, did it's job, but I always wondered if the touch experience would've been better without the protector on the front. I also have an android tablet that doesn't have a screen protector and it hasn't gotten a scratch on the front in two years (even with the lack of care it goes through).
Hope this helps...