simply put, playing a lot doesnt mean youll be able to help develop a good idea, and playing a little doesnt mean you wont. there are more poor card designers than good, but as you said they have on occasion had one good idea. these people are not prime for idea guru as OT detailed, but are the people that would be the ones likely to be posting in the ask the idea guru thread. those who have been established as the better card designers would be more suited for the role in aiding others in providing feedback on card ideas than those who just know how to play the game.
if we use a 1-5 scale for card quality, with 1 being very bad, 2 being bad, 3 being decent, 4 being good, 5 being great, most card ideas made are 1-2's, and with water being a tricky element to design for, its going to get more than its fair share.