Originally Posted by
goalie
Some partner airport lounges may deny you entry/access if you don't have your own airline's frequent flier card (i.e good luck getting into a LH lounge without your UA Milage Plus Card showing your status

)
That's why I carry my BA card. It's the only one with Elite status on it. For the rest, knowing the number so I can make sure it's on the flight record or hotel reservation is enough.