Continuity is a big part of why you only use one QB. Also, it's difficult to keep two talented, experienced QBs on your roster at the same time. Cap economics just don't make that an easy thing to do. You build your offense around this scheme and what happens when you lose one of your QBs to free agency in a year or so? You end up having to get another talented QB who can come in and run your offense. Just not easy. Lastly, in reality, you have to have three talented QBs on the roster at any given time. Because you need two guys to run the offense, if one gets injured, you have to have a guy who can step right in as a back up and another guy who can then replace the backup. Just doesn't make much sense to me.