Several of my bikes have minor dents. Any bike that gets ridden enough will pick up patina, and sometimes that includes dents along with paint scratches. "Previous owners" in general aren't known for babying their bikes or caring about them too much, otherwise they'd still own them. Barn-finds (or storage-room-finds like my Raleigh) often pick up dents from their storage history. My least patina'ed bike is probably the orange powder-coated Bertin, and it may be the worst rider. Even the super-bikes aren't flawless. None of the dents bother me, none are structural.

I couldn't say how that affect value. Someone looking for an A-grade example or a frame for restoration would certainly walk away. Someone looking for a good bike to ride but who doesn't understand frame dynamics might be afraid of it. For most of the rest of us looking for a rider-collectible, then it is less important.