There are so many exceptions it would be a clusterfuck of a rule. The blister situation. A guy who's on a pitch count coming back from injury. I don't see how it could work. If MLB really wanted to promote length from their starting pitcher, the only practical way I can see to do it is to tie your DH to the starter. Jose Quintana starts the game. JD Martinez is his DH. So long as Quintana is in the game, so is Martinez. If he gets taken out, Martinez is similarly out of the game. The next time the DH spot comes up, the hitter represents the current pitcher. If Martinez spot comes up while Reed Garret is pitching, now you bat Ben Gamel. He's tied to Garret the rest of the way. In other words, not every reliever gets a DH, but every DH turn at bat is tied to the current pitcher.