It's all about sound quality for me. Over the years, I have more or less identified the sounds that I want a call to produce,...those sounds that will consistently pull gobbles out of birds. If a call starts to lose that sound,...and more importantly, if it starts to fail to get responses from gobblers,...I replace it. Of course, that's easy for me to say because I make my own calls (which I advise anybody that really wants to be the very best they can be with a mouth call to do)
In addition, it's all about having confidence in a call in a specific situation. The mouth call I will use for early morning tree-call type talk is entirely different than a call I will use after the birds have been on the ground a while. I want different tonal and volume qualities in those situations,...and for me, I cannot achieve those differences in the same call.