I used a matte black incase slider on my 3G for approximately six months. Very solid case, however it did scratch the bezel quite a bit and kind of pi$$ed me off. The body of the phone was pristine however. It did save the phone from at least one fall to a hardwood floor with zero damage to phone or case.
I think that is the cost/benefit tradeoff. Small particles of dirt / dust get into the case from pockets, bags, etc. That dirt is highly abrasive within the case especially with the tight fit. Personally, I'm hoping for the avoidance of a catastrophic fall and will have to deal with the scratches
As an analogy, car covers present a similar problem...