I've heard the slippery slope argument about "why just 1 DH, why not two separate squads of David Ortizes and Brendan Ryans?"
I've not heard this. I think there's a very good reason to reject the slippery slope argument though - the fact that we have 42 years of AL history without any movement (even from crackpots) towards more than a single DH.
Personally I just don't like watching terrible sports, and a pitcher batting is terrible sports. Pretty much equivalent to position players pitching, which only happens when there's no other alternative. In my perfect world, pitchers batting would be the same - like you only see a pitcher bat if there's an injury in the 18th inning and you don't have anyone left on your bench.