I didn't explain my idea very well (or maybe you were responding to the original idea of skill improvement not tied directly to training).
I would expect that teams would still train their inside players in mostly inside skills. But what I was suggesting that you would pick a training regime and a position focus. What would be different from the current system is that some training would go to all positions played.
Let's say that you were training SB.
If you trained SB - C the training might be distributed:
58%, 25, 10, 5, and 2 (for positions numbered 5 to 1)
So even your PG would get a little bit of improvement in SB.
If you trained SB - C/PF the distribution might be:
35%, 35, 18, 9, and 3
SB - PF: 25, 40, 20, 10, 5
SB - PF/SF: 16, 31, 31, 14, 8
SB - SF: 10, 20, 40, 20, 10
...
SB - PG: 2, 5, 10, 25, 58
Note that SB - PG might not be as effective as SB - C, simply because PG can't utilize SB as much, and thus don't get as much in-game reinforcement.