I've been reading a few old books, mostly interviews of great pitchers as I am putting together historic rosters for a PS3 game called MLB 14:The Show and so I am researching all the pitch types - what did Addie Joss through? What did Herb Score & Sam McDowell throw? How about Don Drysdale and Sandy Koufax? Bill James & Roby Neyer's Guide to Pitchers has been incredible helpful - an interesting book/read if you want to know who had the best particular pitch over historical periods and all-time.
All that to say, my research and study has led to many interviews where pitchers pretty much said they had 'this pitch' or 'that pitch' and the like...'from God' or 'the baseball Gods' or 'I was just meant to pitch'. And it would seem to fall in line with the number of injuries. Some just are blessed with the physical body to be a pitcher - arguably the hardest action on any professional athlete.
The other thing I noticed was that most pitchers didn't throw a curveball or their screwballs or any thing with heavy wrist action but maybe 5-15 pitches a game. It was mostly a 'hop ball' (rising FB) or a 'heavy fastball' (sinker) or a what we know today as a cut fastball as well as lots of changeups to palmballs. We also saw a LOT of knuckleball pitchers. The curveballs, sliders, screwballs, etc were mostly used 'only as needed' type of scenarios - up until Sandy Koufax and Bert Blyleven types. JR Richard and others would throw slider, slider, slider, fastball, slider, slider, fastball, slider, slider....done at age 30.
Then the 80's we see Roger Craig and others put a greater emphasis on splitters and forkballs and sliders and so forth. There were mostly rotator cuff injuries before, but not, the elbow injuries are becoming more and more prominent (more so than shoulder injuries even).
That said - baseball is becoming a pitcher's game. How many arms will teams need though? Will a MLB team need 20-25 pitchers per year to get through a season, as injuries would take their toll each year with the idea of having to have pitchers with great sliders, curves, etc. (heavy wrist action pitches)
The trick, I guess, would be if we run out of arms - e.g. not enough pitchers to meet that 20-25 pitchers per club needed?
Versus having a team of 12-16 pitchers who have command, offspeed, cutters, fastballs, and another non heavy wrist action types of pitchers?
Does the former (heavy wrist action pitchers) have enough supply and provide lower ERA (or opponent run production) than the latter? Most likely. So it's probably not good for individual pitchers, but would still be advantageous as it gets a better result in the box score.