It's not just a matter of principle.
I know what I can do well. Suppose someone asks me to do something I don't do, or in which I don't have as much experience as I would like. I then have to guess whether I can do it, based on what I already know and have done, or whether I'll do a job *I'm* not happy with -- because if I'm not happy, there's an excuse for them not to be happy.
Which is one reason that one of the last commissions I turned down (a while back -- I've not sought photographic commissions in years) was 3 weeks' architecture in Saudi Arabia... Still wondering about that one, too.