There are scenarios where Batman has stated that he is not cut out to be a full time member but agreed to be a part time member of the Justice League
I am thinking - should JL be a showcase for the lesser-well-published heroes and heroines and Superman is called upon when the League really needs him
Else the writers have to "degrade" several of his attributes
Intelligence - because Batman has the copyright on that
Speed - because that is a Flash trademark
Will Power - oh no - that belongs to the Lantern
On the same lines how would you feel about WW sitting out the JL as well - but called upon for the more magically inclined foes perhaps -