Knowing the elemental/abyssal/infernal/celestial are good for summoners. Especially if the summons don't speak Common.
For total Polyglot status? You can seriously freak out an Aboleth, which would be totally worth it. Maybe even ask it for some of its mucus ("Wait, you're telling me you people want to buy my snot? Sure, take it. Whatever floats your boat, freak").