Tiger Moth:
I would not be doing it for the hours or to keep "my hand in" because I don't need the hours and the experience would not be relevant to airline flying anyway.
It is easy to dismiss FI flying as having no link to airline flying. I probably thought along the same lines when I was instructing full time. However I have been in the airline world for nearly 5 years and my FI experience has proven to be a fantastic asset since I am now a line trainer and my FI experience enabled me to get involved in training within my company. I have kept my FI rating current and still instruct and still enjoy it. I was told a few years ago to keep my FI rating current which I thought was daft... now I see the sense in what I was being told!