I certainly don't think that protective filters do much for protecting most of the time, and I don't use them except in specific unusual (for me) circumstances. And I am sure that the modern coatings are pretty tough, from experience. Thom Hogan's pushed it further than I have, but I've had stuff hit lenses too.
On the other hand, I do NOT think that any quality filter should make an image "soft" - they are supposed to be optically flat, and in my experience, it's a pretty poor one (and probably a really cheap one) that is not optically flat.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!