If you are throwing side arm you get a different suite of pitches than over the top.
From over the top or 3/4's you can throw just about any pitch: fastball, sinking fastball, slider, splitter, curve, change up...most importantly your curve moves verticially (down).
When you drop down to side arm, your breaking ball will be pretty flat, and if the batter bats opposite from what you throw, he will get a very good look at what is coming. Most sidearmers are relievers and are either set up guys or specialists.
I think this is the fundamental question of whether to drop down or not is what are you going to do with your breaking ball. I also think a good change up is a great benefit, Ian.