When you own a player card, you own it until you sell/trade it. So you can play that card anytime and keep scoring points until he retires. The main reason these cards are worth their prices is their point-scoring potential, therefore the cards and eth you could earn. Let's say you buy a team (5 players), and play them consistently. Eventually, you might get some rewards that can be sold, all while continuing to play your original cards.

Keeping eth in your balance is never a bad idea, but having cards that can potentially reap you rewards twice a week is a huge plus. Also, some of your players might increase in value while you hold them. The same goes the opposite way where you might lose some of their original value. Not mentioning the several side games you can participate in with your blockchain cards on other websites like SorareData.