I'd achieved a decent level of success, but can honestly say since investing in sports psychologist and massage doctor about two seasons ago, they are hugely worth it. My performance has shot up dramatically since I can rely on having my starters usually start both league matches (or if I want to focus on the cup, one league game and one cup game).
Before that, I'd either have to make the decision to have them start one game and backup 1-2 games, or run the risk of dropping GS badly if I did start them twice.