I agree that the instrument itself dictates its 'lendability'.

My attitude is that I depend on my instruments for income - nobody asks a garage mechanic if he'll lend you his toolkit...

I say 'No, I don't lend them out' - having said that, I lent a good guitar to a girlfriend over a year ago, and told her she could keep it - even after we broke up...

Suddenly I get this horrifying spectacle of hordes of women hammering on my door, just so they can get their hands on my instrument...