Comment by Zak

Alkaline has a tendency to leak electrolyte when stored in devices long-term, especially if used intermittently, even more so if the loads are at the upper end of what alkaline can handle as they are in many modern flashlights. The electrolyte is corrosive and often results in a broken device, which is exactly what you don't want in an emergency.

Li-ion's self-discharge is pretty low for a bare unprotected cell, and a flashlight with a mechanical switch consumes no power when off. One must take care to avoid short circuits when handling such a cell, but modern Li-ion flashlights have over-discharge protection, so that's the main safety concern with a single-cell design.