The two positions, catcher and pitcher, are very risky. There have been many that have done it, though. Matt Wieters actually did it in both high school and in college.
Most catcher injuries are something other than arm injuries. Most pitching injuries are arm injuries. So catching and also pitching carries a lot of risk for injury. And when the body is worn out, the chance of arm injury increases.
I have a hard time believing the throw from the catcher back to the pitcher is the same as 1/2 a pitch. Most of these throws are a simple snap of the wrist and with less effort than a Batting practice pitch. There are BP pitchers that can and do throw hundreds of pitches every day without suffering injuries. Obviously the throws down to 2B are different than the toss back to the pitcher.