Nellisir
Hero
Doesn't occur to anyone that bewbs on a bird is kind of silly, anyhow? Makes about as much sense as having them on a reptile. Hello, not mammals, ergo no mammary glands.
Still, depictions of a harpy or a succubus are not actually in the 'stupid' category. Assuming you can get past the whole avian/mammalian biology thing by imputing it to magic, anyhow. If it makes actual sense for them to be showing boobs as opposed to it being incredibly foolish for them to be doing so, at least you're not demeaning the character being depicted by showing her as stupid.
I think harpies with boobs is a slippery argument. The dragonborn were a result of WotC creating a new race whole cloth. Boobs were silly. The harpies are mythological creatures that were converted for use with D&D, and were often depicted with breasts (but not always). Biologically unnecessary? Yes. But true to the original depictions of the creature. There's room for interpretation, and even for eliminating the boobage in that case, but not via a science argument.