i believe this suggestion willlead to much more farming then it already is, because when you have an salary cap your centers will need a lot of of sideskills who nearly have no effect on salary but 4-5 off them will be better tehn one centerskill.
But to train them and keep them in shape, you have to play him out off position and when you play your best player who is an center on SG to give him outside shooting training and then try to manage that he don't gets more then 65 minutes - you won't play succesfull and orientated for team succes. BB-Charles who was mentioned as a farm here, plays Grubbs a lot, on the right position and wins his games from time to time.
This is why the request of better training for SFs could be considered...
I don't agree with the salary cup idea,the limit is all you can pay to sustain the salary of a player,if you follow the rule
And I think that in the future,many centers traineers,will train more outside skill,as n reality happen(a player like Dwight Howard is yet very strong in inside skills,but lacks of passing,outside shooting and free throws skills which he's training a lot now)