it isn't about an ingame advantage for supporters, the main purpose of a PL is the additional fun to play with your friends, people from your federation or opponents you choose.
Having the chanche to test is just a sideeffect, and i am quite sure the advantage you gain is still bigger, the opponents may gain also information about it but normally just you should know the skills of your players ... And just because you play Princeton in the Pl don't mean that you will play it this week in league ;)