AArgh! You never put tub-surround or shower tile up with mastic! Thinset mortar only. Mastic-based failures are far more prevalent than successes.

I think there is some truth to the idea that in the old days with 1 1/4" thick mud job tub surrounds, the caulk of marginal quality back then, it was a "wash".
Remember grout lets moisture in AND back out again, it's a revolving door, so caulk at the bottom is not trapping vast amounts of water because it can evaporate via alternate routes.
Casey