I actually disagree. If you are buying a player mid-season, he won't retire after the season. If he were retiring, he could not be sold!
I have found this a great way to add quality to my team mid-year. I can get older players cheaper (and, most importantly, with fewer tokens). If there is a 33 year old 5 star player for example, he is usually very cheap for the quality you are getting, and he'll still be a 4 star player next year.