We have to ask our selves this when we consider adding Jason Taylor. Does he provide an upgrade? I think he does, plus he's leadership material.
Jason Taylor may make us only marginally better, but if we get marginally better at most of our positions, then we are vastly better. That is the goal, to get better.
I don't think we avoid someone because he is not the MVP caliber he once was. I think we should target players who make our team better, regardless of their age and stage of their career.
A better team sees no age, it sees production.