a) It's all depends from your sources. If you have extra sources (farming, buying, downloading app in mobile), you can buy young fast trainers and make them easily 6*-7* players. This can give a small advantage vs other teams.
Then you can keep them for some more seasons if they are good performers.
If you don't have many extra sources, there is no reason to bother with.

b)
The free one is useless except for lowering quality at the end of a season and the higher one could be useful but often a waste of time if you have a squad like mine.
or sell them for some good money.
Also if you have extra sources, 4* academy players are useful for higher levels where the market is "dry" .