Lithium Ion performance and value

[RITMarketer]

I've seen it mentioned in one of these forums that Lithium Ion batteries perform better when kept at a high/full charge. Can I get an overwhelming "yes this is true" from techheads, or have someone explain to me that this is the truth of the matter? I kinda wonder how it is that a battery will "perform" better at times. I've always perceived it as "it works" or "it needs charging/is dead".

I do understand battery memory (if not the technicalities of it), so comparison with Nickel isn't necessary, just confirmation that LiON perform as well/better with a constant charge and a layman's answer to why?

Thanks
Pat