I renewed my FI rating in the last 3 years, and just managed to scrape 12 hours P1 in the last year to renew my SEP. No prizes for guessing that the day job is on a 757. I'll probably renew it again in 2011
Quite a few airline pilots who came from a GA and instructing background keep the rating current, or renew it after many years; however they don't do an awful lot of instructing.
Several of the Capts I've flown with over the years have specifically renewed the rating to teach their children to fly.