Do they not let you add to the dictionary? Honest question, I've only used Swype, but you can definitely add things to the Swype dictionary and they work just like any other word after that.
They do, but in order to enter a custom word, you have to switch typing modes to a full-screen keyboard, which is a 2 second action. With sliding keyboards, you just start pecking because you already have a full-screen keyboard.