Why should you? They want you to do it, which is already reason to be suspicious.
It makes you all the more emotionally committed to buying a car whose immediate feel you enjoy. You might save a few hundred dollars on the bargaining by refusing to take that step of commitment.
Furthermore you might expect that every plausible new car can in fact survive a test drive from a potential customer. Let others test drive it for you.
And let’s say you didn’t so much like the test drive. Is that a bad sign or a good sign about the car? Does your dislike very well predict you will dislike it a month from now? I doubt that. In fact if you are somewhat typical and others dislike the test drive too, that might mean the car is all the more a bargain. And you are letting a mere mediocre test drive persuade you away from exploiting that bargain.
I readily admit this advice does not apply to very tall people and other outliers.
Question: to how many other spheres of life might this reasoning apply?