I think that apart from everything that is said here,
and I agree that it's all positive, and I as well would like to train just the way I want,
the BBs choose not to change it, and might have purposely choosen this system in the first place...
in order to prevent new players from drowning....
BB lets you choose from a variaty of trainingtypes for diffrent positions. I think they did their best to make sure the most important things where included. Yes for several very specific situations you could use other types/positions still, but in the end the trainingtypes they offer, will probably gain the most overall (players train the skills they most use, even if they do use all skills , some are used most, so training those would benefit most in the end).
There is also the scrimages in which you can play specific players at a specific position if you realy want a specific player to train a specific thing.
Last, but not least, I'd like to add that even though you might not want to train your center while training your PF (or any other combination you might think of), but in the end I don't think they are all legendary, so they will get better, even though you find that they are good enough already, they will become better still, that's great isn't it?
(ofcourse I know one would like to put some 18yr old in the trainingspot instead of your 28 yr old center to get the max out of training, but hey... sell the 28yr old and buy a 18 instead if it's realy that bothering)
If 1 player is realy upsetting your trainingsceme that much, I'd sell him and buy an other that fits more into the scheme, rather than trying to have the system changed. After all, it's the same for all. And we get a good variaty of choices for training.
This does not mean I wouldn't like to train exactly what I want and that I also would like that option, still I think I find it better this way, if just only for the fact to prevent that people who don't realy know what they are doing go train skils for players they won't often use.
They are not your friends; they dispise you. I am the only one you can count on. Trust me.