Do you know what happens to humans? they work ok at start...then, as time passes, people get tired (others have issues like stress) and, although still working, they aren't working like they did. Then there comes a time when all we want is to rest, sitting in our arses all day long.
batteries are the same...Since there aren't perfect systems, they get "tired" of working and won't last that long. They are disposable pieces anyway.
In my case, still running eGo batts 3 months now, with almost everyday charge, they are perfect EXCEPT one of them that is suicidal...keeps falling from everywhere (table, pocket, hands...) and it's all messed up..lasts me 2 hours or so.