The argument for the DH is that ultra-specialization of pitchers already happened without it. The DH is a response to that specialization rather than a cause of it. Pitchers as all-around players was the original idea in the early days of the game, but after the switch to overhand pitching (the original rules called for underhand, as in "pitching" horseshoes), the position became specialized pretty quickly because the pitcher has vastly more impact by how well he can get batters out than by how well he hits over a few PAs. By the start of the 20th century, teams had already stopped caring how well pitchers hit and basically only selected them based on their pitching ability. Even before the DH, it's happened that a pitcher has gone an entire season as a starter without getting a single hit and it never jeopardized his job.Famous Mortimer wrote:I find the increasing ultra-specialisation of sport boring. It just smooths out all the fun little wrinkles and inconsistencies that make sport what it is. Perhaps if "we" expected pitchers to be better hitters, they'd practice more, and the all-rounder would be a more common thing than a random sack of flesh who can throw 101.
You occasionally get a pitcher who can hit, but teams view it as a bonus and don't really decide playing time by it. Micah Owings for example spent the latter half of his career in the bullpen (in the NL) even though he was a good enough hitter to also pinch hit because his teams preferred to have a marginally better pitcher at the back of their rotation rather than get Owings' bat in the lineup. MLB's history pre-DH (and the NL post-DH) indicates pretty strongly that even when pitchers have to hit, teams still prefer the guy who can throw 101 but can't hit to an even slightly worse pitcher who hits well.