As someone that regularly stands in the middle of the road directing traffic I want to point out that there are an awful lot of drivers out there that take little to no interest in whats on the road ahead of them.
My arguement is, that any method of highlighting that there is an object there which may move is a positive step, and Arnold, I know you may think that a moving car is always easy to spot, but to be honest that's not always the case. If your out on in a semi-rural/rural area and a car is coming at you along a line of parked cars with the sun behind it, it can be almose impossible to spot which is which. I would say especially in winter with the low sun, but it can be just as bad in summer with glare from reflections and other distractions.