They clearly didn't have a warehouse full of R spoilers to use up. :eusa-think:
I agree about the quads, my preference too, but I think If they had put an R spoiler on, it would have looked too much like an R lookalike. I think this would have somehow de-valued actual R's, at least in street cred if not monetary value.
If they really needed to make a run-out model, I think they should have thrown a few more bits at it to make it stand out from the rest.
They could have given it quads as standard, maybe lowered it slightly, and put a bit of body kit on, so if you saw one, you would instantly know what it was.
Just my thoughts.