Whatever issues LaBrie's may be having with his voice as he ages he used to be in the upper echolon of rock/metal singers imo - right up there on the Tate (well, back when he was good), Dickinson, Halford, Dio, Coverdale, et-al, strata - and having other people singing on DT albums who didn't blend well with his voice due to a technical disparity and a clash of colour/timbre was certainly a choice they made.
It worked out in a couple of places but overall? No thanks.