I bought this guy last season
(16552989).
When I bought him his ID was awful, and for this reason I was able to get him fairly cheaply.
After a season of training, I had got ID up to average. (This was whilst two posiition training with some IS and RB).
I was able to sell him a year later for $1,100,000. I must stress that all his secondary skills were good (eg JS, OD, PA etc.)
If I had of kept him I'm sure he would be pushing into my first team by now. I think as long as the player has good abilities in the other important skills, it is ok to train a player with less than mediocre. But you have to focus on their weaker skills if you want to make them a great player.