Just realized that mine turned up missing. At first glance, it doesn't seem very important -- the cap itself doesn't make a tight seal, meaning the only purpose it could serve would be to protect the contact pins from physical damage, and the pins are recessed, making such damage unlikely. Suppose it could also serve to protect the inner electronics from electrostatic discharge.
I'm mostly inclined to believe it's a cosmetic thing, in which case I'll get along well enough without it.
-- David Barts Portland, OR
