I wouldn't be afraid to use unprotected batteries as long as I had a protected charger. Overcharging is what is dangerous. Over-discharging can ruin the battery, but won't cause it to explode or catch fire. You just have to change batteries as soon as they show signs of weakening and you won't over-discharge.
In a sense, no LI-ion battery is actually "protected". Not internally, anyway. The so-called protected batteries have a protection circuit PCB attached to one end and a wire that runs to the other end. This is usually all covered by a plastic sheath so the end user can't tell without looking closely. With unprotected batteries, the same protection circuit is built in to the charger, so there is still protection against overcharging. The only real advantage to protected batteries is protection from over-discharging.
Edit: I guess there is a danger if you use the unprotected batteries with the wrong charger, but assuming you have the correct one I still wouldn't be afraid to use them.
TT33