I gotta say, I agree with a lot of what the others were saying in this thread. A few years ago, I wanted to see how easy it would be to "cheat" the game. I wanted to test how bad the AI would really be (Easy mode anyone?) So I decided to use one of my worst pitchers with the intention of getting the mirror of truth once I made him the best I could....(yet probably the only one who could play a realistic number of innings as a starter)
So here's what it looks like: (I forgot the numbers for stats.) Stamina: A Control: G Knuckleball: 4 Knucklecurve: 1 Sinker: 3 and 82 mph (first year starting stats)
By the end of year 2, my Stamina (A) and Control (G) stayed the same....but as for pitches well:
Knuckleball: 7, Knucklecurve: 3, Sinker: 4, added a new pitch Splitter, got that up to 2. mph 83(?) [Im sure you can guess which pitch is my favorite

]
And that's not even counting all the abilities I stockpiled up. Naturally, I got bored of my success mode mylife player, because it was broken beyond anything. Sure, I suppose you could say my selecting easy difficulty was the reason, but you would think the team could score some runs. One time I had this incredible pitching duel with C.C. Sabathia. 13 perfect innings thrown by me (remember this is easy difficulty), and my guy finally gets tired, coach pulls me, and we lose in 16 innings by a score of 1-0 lol
So with that said, my rookie season I won every pitching award possible and posted 23-1 record (way too many no decisions) and the second year, which i was a couple of weeks away from finishing I already had 25-0 record with under 0.5 ERA (again, I was "cheating" with easy mode) Nonetheless, I can't tell you how many games finished 1-0 when I was pitching. The problem with the game was that the scoring was progressive in a way, when one team scored a bunch, some kind of catchup logic would kick in and the other team would keep the game close. On the off chance I would give up a run or two, my team suddenly learned how to score runs.
When it comes to being a fielder, your player develops too quickly. I've made numerous power hitters with the intention of them being career DH's, only for them to build up their speed stat up to a solid D by year 2-3...and the coach plays me in the field after that.
TL, DR;
PITCHER DEVELOPS PITCHES TOO FAST, POWER HITTERS BECOME THE MODERN EQUIVALENT OF PETE ROSE.