I wondered about that too, and one thing that occurred to me was that if they get popular, they've got access to combine data about their users across all the source and sink services the users use. That is, they can get all my readily available Facebook data (friends, likes, etc.), my Twitter feed and follower/followee/favorites lists, etc.

I doubt IFTTT is doing such data fusion today (their userbase is probably too small to make it worthwhile), but I wonder if that's the eventual plan. To advertisers, that sort of data could be immensely valuable for ad targetting.