IMC, it's been coming up pretty regularly recently. Recently the PCs had to negotiate with some tribes of orcs and if a couple of them hadn't been able to speak orc, it would have gone very differently.
There was a funny moment in our last campaign (I wasn't DM-ing) where the PC party was heavily disguised with magic to sneak into a certain guarded area and discovered to their surprise that the guardian was a large red dragon. Unfortunately, the dragon decided to talk to the guy polymorphed into a hill giant, and that's when we realized that he couldn't speak Giant. Luckily the mage fired off a disintegrate (3.0) and the DM rolled a 2 on the dragon's save, but that's a story for the "auto-kill spells and effects" thread.