I'll second Betweenrides I only give/lend out single cell lights. I generally don't bother with protected though as not much can go wrong with a single cell.
While having 1 cell at 3.6 and the other at 0 is dangerous it does happen and rarely will lead to an explosion. This is the second documented case of what should have lead to reverse charging but didn't. (The other one being mine) I think even good unprotected cells will not reverse charge. My current theory is the thermistor in them (magical little thing 
) will prevent them from reverse charging because they would heat up from that. The real danger is the crapfire cells.