Li Ion batteries have to have complex internal circuitry included though to prevent them from quite literally exploding. They have strict working conditions that they must stay within. That is why they will probably never come in regular battery sizes.
NiMH batteries are really much safer in comparison, and not that much worse in capacity. NiMH is even better than Alkaline for many uses, such as digital cams where Alkaline's quick loss of voltage makes them seem to become dead a lot faster. NiMH has a much smoother voltage curve (starting lower though).
Rechargeables shouldn't be used in devices that will totally drain the batteries though. Flashlights, for example. Electronics just stop working at a certain voltage whereas flashlights are just resistors that will drain everything out of the batteries.