I started off with that idea too, but the thing is the rest of my team was developing much quicker than my trainees (I was able to afford better veterans quicker than my trainees got better). If you do 2 position training, you can only train low-medium potential trainees, and they'll never be a key contributor to your team. Why not buy such a player instead?
If you train to sell those players, they won't generate that much money because they'll be relatively old for the skills they have. I think it's better to train better guys quicker, or even the same trainees. For example if you train low potential guys for like 2 seasons 1pos or 3 seasons 3 pos, you can sell 9 19-year-olds after 9 season with 1 position training, or 12 20-year-olds after the same time. That extra year on 9 players is worth more than the 3 extra players I believe.
Anyway, the reason why I train is because well trained players (good secondaries) are very expensive/rare on the market. The difference between a well trained player and one without secondaries isn't that bad for lower potential guys, but as you get higher salaries you want more skills for it because you're paying a hefty sum and then you don't want limited players. If you train low potential guys (or 2 position training), you'll never have use of them because you can buy better players by the time you're trainees are done.