Some people may decide to come back a year or 2 later.
I think leaving the memberships intact is a good idea, you never know when someone will decide to drop back in and post something.

I haven't posted on a few forums in months, some over a year, I might drop by in the future and have a read and see something I want to post on... oh whoops that's right I got that e-mail that said they were deleting me if I don't post... guess I'll go to another site.