WWW,
Looking back I know I was in the former camp and I am now cosily in the RHS. I recall colleagues from the latter group and not one ever made it.
Looking back to when I was a PPL student, my experiences were exactly the same. I used to get annoyed - the good instructors got jobs quickly and left; the bad ones stayed around. Looking at it as a student it seemed thoroughly unfair! When would I get a good instructor who stayed? I finally did...he got a job the day I finished my course.
He taught me a lot more than how to fly. He taught me, by example as much as anything else, to do my best at EVERYTHING I did, and never to accept second best in myself. A useful lesson.
And even taking it from a selfish point of view, why not? So you're an instructor who doesn't want to do the job, who longs for that airline job, who doesn't even like students. You still have to be there, to do the instructing. It takes no longer, nor is it harder, to do a job well than to do it badly. A miserable job is far less unpleasant when you're putting effort into it than when you're counting the hours. And who knows, being well thought of migh stand you in good stead - that next student might have contacts in the airlines, or know someone who knows someone who...
How can you tell if you're recruiting? I think you can. A person's basic attitude shows in everything that they do. Someone who does their best at everything, who tries to be a professional, WILL dress appropriately, turn up when asked or explain very clearly why they can't etc. That person will try to be a good instructor - and will be, even if it's not what they want to do. The one who doesn't will never be any good at anything, because they'll always stop trying when the going gets tough.