All battery ages, and have a lifespan. Even NiMH AA batteries or your any other Li-Ion batteries. The gauge is for you to know the battery life is deteriorating and will not behave as good as before. You can safely ignore the warning if you'd like.
I concurred, my en-el4a had reached "4" for the last 2 years yet I still manage to continue shooting and complete given tasks and fill-up an 8gb CF card of raw images without swapping the battery.
But then, I don't chimp too often.