I think you're going to get different answers depending om who you ask. Imo, the answer depends on what your plans are for next season.

For example, for the coming season I'll need to replace my DL and MC. Since I wont have any backup players for those positions, except one MC from the Academy, I need to secure those positions. I have two choices: sell my current players this season and find replacements now so I have two players ready to be trained (next season), or wait till next season and acquire those positions. The pros with the former is that I 1) don't have to fight over those two players with all the other managers at the same time everyone's looking for new players and thus pay to much Tokens for them, 2) they'll lose 20 % in quality which is good as I'll be able to train their white skills even more because the lower grey skills next season will decrease the average quality for drills I want to train (this can ofcourse be seen as a negative as well as the white skills will decrease as well). The negatives are that 1) their attributes will decrease by 20 % and that they'll be 1 year older and be marginally less efficient in training. A third option would be to not acquire the players this season and utilise the academy player for the MC role until I find my replacement next season. But since I plan on releasing the academy players (since they will increase my overall quality for the draws next season), I often opt to acquire my replacement players so they'll be in place for the coming season. I don't want to spend Days at the market trying to find fast trainers when everyone else are doing it as well.

This season I've already acquired 3* DL and MC to replace my current players. I use the non storable rest packs to train them which will free up resourses to power train other players next season.