The best example to understand the "truck sucking" is if you run Extreme Luge in MTM2. Towards the end of lap 1 you will witness the phenomenon.
http://www.mtm2.com/MTM2/Tracks/tracks. ... treme+lugeOther than that, these ramps/wedges were used if you wanted for the truck to run over an obstacle (like those dinosaur bones) in case the normal square box was, for some reason, impractical.
Edit: Also I think that ramps were always interacting with AI trucks. The thing with the normal collision box is that if you are in the vicinity of the box, the AI will interact with the box (meaning it hits the invisible wall) but if you are further away the AI and the box will not interact (the AI will drive through).
I've used this effect in Arizona Dreaming to create shortcuts for the AI. If you are right next to them when they try to take the shortcut they will hit the invisible wall.