There wasn't any sort of "clan cleansing" but there was:
There was a zero-post forum member cleansing, but I don't remember a clan cleansing.
Actually, I think we've done something along these lines, once before---years ago.
Correct me if I'm wrong.
So they take up space, it doesn't effect you in any way.
Just hazarding a guess, but I would imagine too many practically inactive clans is just unnecessary load on the server.
If there was too much load on the server, Oliver would have already put a max limit of clans.
Currently running a test (on a private LoGD server), and I have yet to run into any strain.
Clans don't seem to take up much space.
But, as I am not an expert of the technical side of LoGD's, I could be wrong.