I've been using my N900 without lens cover (Mugen cover) almost from the start. No scratches, no dust, no fingerprints. But with Mugen cover the lens is somewhat indented which may help. Didn't have problems with Nokia N95 8GB either without a lens cover. I usually carry a phone in my pockets, of a jacket or trousers.

I would do the same trade-off again, let go lens cover for a bigger battery. I'm fine with the function when I push the camera shutter button down, a camera application starts.

I wonder if there is studies, is a lens cover in mobile phone really actually useful for protecting the lens or not. It may be just a feeling it would be.