I don't think it diminishes his legacy at all. In 20 years, people won't even remember his time with the Clips.
Switching sports, is Willie Mays' legacy diminished because he finished his career with the Mets? Do we think less of Babe Ruth because he played for the Braves? What about Bobby Orr playing for the Black Hawks?
Or, what about Dave Cowens on the Bucks? Is his legacy harmed?
Pierce is a competitor who likes being around the game. If he wants to hang around past his prime, good for him. He's earned that right, and history won't judge him poorly for it.