How come I never see anyone using coin cells in their robot?
There is at least one on the net - a tiny line follower, but it's rather boring.
Aren't they pretty small and carry a good amount of voltage?
Couldnt I stack two coin cells and I would get the 6volts I need that I get from my 5-cell AA NiMH pack?
Voltage is not enough.
With less than 200mAh in LiIon button cells, you need very low current motors for any kind of fun.
Besides, primary LiIon button cells cannot deliver very high currents and the rechargeable types even less
Much better to use Zinc/Air for small 'bots. The 675 cell holds around 650 mAh (1.4V) in the same size of an AG13, so they are very good power/weight and capacity/weight wise. Alas, they are primary cells.