I don't think there is any doubt that SD is the best keeper at Somerset, and also a competent 'red ball' batsman, but I don't think that is the problem.
My understanding is that SD considers himself a strong 'white ball' keeper/batsman and, indeed, I thought that was the reason he (was) signed.
I suspect that, if he doesn't appear in this season's T20, he will not be here next season.
TB is now out, by the way - 64 off 34; quite a good debut....