Capt Pit Bull,
To use you analogy then we should never pick up friends and family from the airport as we are doing the poor, hard working taxi drivers out of a job as well.
Working ad-hoc for a local flying club can hardly be seen as an attack on the full time Flying Instructors position. As long as the position is approached with common sense and sensitivity as to the concerns of the full time employees and an acceptance that the part time role is one of support it shouldn't be a problem. I made it clear to the company I worked for that I would only offer 'cover' when one of the full time pilots required it.
Certainly, as an airline pilot, I could not be relied upon to be able to step in short notice at any time during the month thus my role was seen more as 'if and when available'. As long as an accomodation is reached as to the specific role of part time instructors and their position within the heirarchy of the club then I don't see why it has to be such an onerous thing.