So if you are terrible at one game and excel at another then you get good at it and suck at the one you were good at.

I wouldn't mind trading my (moderate) ability at VVVVVV to become instantly better at Final Fantasy 9. Though, that's less because of the likely intended reasons, and more because getting better at VVVVVV was FUN and I wouldn't mind being able to go through the learning process again. FF9 would take a while to get good at, so this would shortcut that process.

i'm great at every game i play so i don't need to trade jacksquat, get owned losers!

Why would I trade skills? I mean, learning, practicing and improving the skills in a new game are basically the reasons, why people speed run, right?

Yeah but say you want to lose your skill in one, but gain that same skill in another. You can easily get it back since you already ran it before.