well I think it depends,if you know the person,and you know they are not irresponsible no problem.
for example if I was at maidenhead folk club,and leadfingers wanted to borrow MY guitar [no problem],or if Tony o Neill wanted to borrow my concertina no problem.
If someone was stoiciously drunk,or a complete stranger and not sober then no.
Sean Cannon always used to borrow guitars when he was guesting,and therewas never any problems.