I wouldn't be that excited to loan out a boat to someone that I absolutely did not know at all.
BUT... in the world of kayaking, especially in the class IV/V range, its rare to meet people that don't at least have common acquaintances. If a traveling boater wanted to borrow a boat, and we had mutual acquaintances, I wouldn't hesitate. Experienced paddlers know its a karma thing, and are usually willing to help out, as they know the odds are they could need help shortly. Its sort of the team spirit of helping each other out.
I also wouldn't have a problem loaning out a playboat or a mankboat. Loaning out a creekboat without a crack in it for folks to run down colorado mank is another story though. I would hesitate to do that unless we had an agreement that a busted boat would be replaced.
I did loan out my mankboat to a friend once... who swam, and completely destroyed said mankboat. It wasn't worth much, so I didn't worry too much.
I also loaned out a bunch of scuba gear to a buddy who left all the shit on the beach in mexico after a lunch time buzz kicked in.
I guess the moral of the story is... don't lend out anything that you can't deal with losing. The other moral of the story is that you can never have too many mankboats.