Well, flips actually put housing that oftentimes "real" buyers couldn't buy back into the housing inventory. Or put old, dated inventory that no one is buying - because again, they only want "DONE!" - back into the active inventory. So I don't view what flippers do as a bad thing. When I don't think it's good is when first time buyers - or any buyers, really - get sucked into a badly done flip that turns into an unexpected money pit for them.

ETA: Sometimes properties will not qualify for financing for a "regular" buyer, so unless an investor comes in and updates the home, it just sits there and rots. Examples = non-functional heating systems, damaged roofs, etc.