Well with Nimhs you can radically improve their life (almost indefinite) by only charging from say 40% to 80%, but you of course lose lots of capacity.
I THINK that the reason Li-ions lose capacity is that the battery material actually needs to expand slightly to accomodate the charge (yes I know it sounds silly, something to do with crystal structures or whatnot, not sure). And over time, the material loses its flexibility and can’t expand as much.
This was the basis of that news article a few years ago that said scientists thought they could make li-ion batteries that lasted 10 times longer – the material was more flexible and better organised on a microscopic level.
So if the above is true, (which would tie in with the fact about NiMHs) then yes, you would be extending your battery capacity by not fully discharging it all the time. Also by not fully charging it each time too.