I bought an U21 today for 21 tokens he's 18 years old 37 rating and worth 7M! I used him to replace a retiring player of the same rating on my bench, there is no way in hell I would have been able to buy the same player on the TL for 21t (and no money ) I have spam trained him a bit and he's nearly ready to be 38 rated.
I also got a poop U21 2 star 18 year old for free today (no tokens or money to sign him) so I signed him and sold him for 750k profit.

The free players are always rubbish but if you need a player its very handy, the U21's you pay Tokens for are available for 7days so thats plenty of time to earn enough tokens imo