I could understand that line of thinking if you had to support the old mods, but that isn't the case here. It makes no one's job harder to allow these downloads.
To a degree, it means more server space used and a ton of unsupported modifications. Think about all the developers who've moved on to vB 4 and are no longer developing for the 3.x series. Imagine how little support would actually be available there. Not to mention, vB 2 isn't exactly being patched anymore. That means that many more mods with that many more security holes. A potential, if small, nightmare.