I have mixed feelings on this one.
On the one hand, I like the "design a car" aspect. Its something to do when in the line. Basically, I think of that as replacing the holding room with the movie.
On the other hand, I will agree that "Test" Track has lost its theming. As hokey as they were, cars going thru bumps, brake test and environmental changes are things that are tested. The ride now seems more out of a Tron movie. I also find it interesting that once you're inside the building, the attraction is referred to as the "Sim Track", not "Test Track".
Personally, I would have gone ahead with the design a car aspect, but just rehab or update the ride (i.e., make the bumps bumpy again and fix the corrosion robot) and not "Tron-ify" it.