It's getting up there in years. When I was an Apple girl, my rule of thumb was to buy top of the line and replace every 7-10 years.

Even with a new battery, older MacBooks tend to slow down from software updates and limited support.

Desktops can stretch a bit longer, MacBook Pros can last a while, but Airs usually need replacing sooner.

You could sell it or keep it as a backup—it really depends on how much you could get for it and whether it's more important to you to have a backup. Hope that helps.