That's very well put and thought out. I've had the same experience. If a friend asks me, yeah for sure, but if a random asks I always politely say no, although whether that has any affect as to whether they do or not, is another matter.
I've had one or two be quite nasty and calling me mean for saying no. That's when I then give them both barrels of f'cks and explain why their heads are stuck up their own arses.
Most of my following (facebook that is) are photographers themselves and I've managed to eventually ditch most of those who aren't.