Replace old shower valves when putting in new handles?

My contractor suggested me to replace the bathroom shower valve since we're getting new handles. He said putting the new handles on the old valve have a higher chance of having a leaking problem in the future. Any suggestion?

Here are two photos of the valves

Submitted September 29, 2015 at 11:24PM by Fix_My_Grammar_Plz http://ift.tt/1h7Z9JU