A lot of damage happens around the derailleur and dropout area. For this reason a lot of MTB's have replaceable derailleur hanger. If you're going to do that, you might as well make the entire dropout adjustable and have the hanger machined as a piece with it. It gives you a more versatile frame that makes it useable for both der'd and singlespeed applications and makes disc adjustmnet a no brainer.

So really the question becomes why wouldn't you do it that way?