Pokémon Black & White (be warned: spoilers)

[Phantom Killah]

Being able to forget HMs on the fly can accidentally strand you. What if you went to like, Sootopolis City, and got rid of your only 'Mon with Dive or Fly. While you can get out of such a situation (much easier now than in RBY), it's a pain. As such, the Move Deleters are located to you can fish for Surfing 'mon, or a mon that learns Cut or Fly so you can access other areas.

You make a good point there.

Well seeing as HMs never go away, you can reteach ti fly or dive if needed.

Unless you never got the HM, and just got the Pokemon with the HM in a trade. Then if you make a Pokemon forget surf while you're on an island somewhere or something, you're stuck