The X1 is to GT5 as the Stone of Jordan was to Diablo II. In almost every comparable way. While it's not as big a deal in GT5 that two thirds of the transactions on the marketplace forum are for X1s, the OP is right, it trivializes the value of the car.
Most of the the people trading for these things are just tryhard achievement whores who just want the best car in the game to pwn noobs with.
It's a good idea from PD to make it so difficult to earn as I assume (I've never driven one) it's an incredibly ridiculous car to drive cleanly. I just watched a 3:29.728 Nurburgring run in an X1 and I doubt I could ever run a car like that. The people trading for them will never have a competitive race in them because it takes such a high level of skill to drive it competently. It's just a joke car to 90% of the people out there. Why bother going to the effort to obtain one when you'll never get to experience a close race in one?