Having the plastic is very handy when visiting lounges of partner airlines. The lounge reception staff cannot always know from your ticket or boading card that you are an elite member of a partner airline.
It also nice when checking-in at a hotel. Then there's no doubt that you are a member of the hotel's program even if it's not mentioned on the check-in clerk's screen.
You can even point out that the hotel actually has a program, as I've had to do more than once ...
[This message has been edited by Austman (edited 04-27-99).]