mate honestly the airline pilots that do instruct don't in general steal work away from full time instructors.
In fact I would argue we actually increase there hours.
The schools I have helped out at its always been a case I have only flown when they are short. Quite often all I did was being the supervising instructor and dealt with questions about licensing. Then picked up any check rides and trial flights that needed done thus releasing the full time guys to go and instruct PPL's. Of course if a full time guy wanted a day off I could help out as well.
Like or not people like doing Trial flights with airline pilots and I always had a knack of converting trial flights into full courses when full time. And when doing them as a spare FI I was even better at it as I usually had more time to spend with them.
But its virtually impossible for us to beat the FTL's, mandatory days off can't be used and you can't go over your duty time for the day and screw up the next days work flying.
And to be quite honest after a week of 25 sectors the last thing I want to do is strap myself into a light aircraft on my days off. So I have let my rating lapse.