It is indeed a useful skill to acquire, but it is somewhat dependant on what you're doing. As part of the house band in open mics, it's pretty well essential. It's no good saying to a singer who needs Bb to say 'but I only know it in D'. You don't get asked back!
If it's in a group situation, and one vocal harmony is too high, it shouldn't take half the rehearsal to change the key, write down the new chords, etc. A couple of minutes and it should be ready.
However, if it's just for your own enjoyment, one key will suffice. But it's a great challenge to take on, when someone who's playing with you says 'you know so-and -so song; let's play it in F# just for the craic.'And often the song takes on a different slant because of it. Relatively easy on guitar with a capo...