Yes I never delete my any single referral. Yes it happened to me too. My 1 referral who joined after my joined in DEC 2011 he/she clicks 24 ads a few days ago.
While here on forum I can see many members registered months and months ago, but having results only in the last couple of months, it means they found reasons to return and became active. I also have half of my referrals inactive for long but I don't consider this a reason to clean them out.