I never purge my cards because I have a fairly small collection so I doubt that I will ever run out of room. If I do then yes I will purge my card.

I have saves from my PS1 games that I haven't played since I got my PS2 in November of '01.

I guess I do clean up my card once in a while, once I see that the card is filling up with saves from my game rentals then I will delete them.