There you've hit the nail on the head. This becomes especially evident when playing more than one chanter/reed combination.There are no good reeds. We just learn to play the bad ones.
There are so many adjustments the player can make - even while playing - I think it means there is a wide range of "good" in judging reeds.
I think this is unique to this instrument. I'm also thinking of P Sky's post some days ago where he discussed adjustments the player makes and how an excellent player will be out of tune at first when playing someone else's pipes.
I bet I'm judging my new reeds by how they compare with the one I'm used to playing.
Dave Jones