Wow. I had no idea bicycle lighting was such a volatile subject.

Anywho...I use two Li-ion lights, one on steady to see and the other on flash for conspicuity. If the steady light battery starts getting low, I switch the roles between the two lights. This gives me plenty of run time and redundancy as well. It's also very easy to switch among bikes. For MTB, I put one on the bars and one the helmet and set intensities as appropriate for the current conditions.

Agree many dispose of batteries improperly, but that's pretty much irrelevant to your choice of power source because you're free to do whatever you think is appropriate with them.