Multiple throttle bodies afford a combination of increased performance and better throttle control- the latter being less of a concern on a car versus a motorcycle.
Another point is that motorcycles, having so little rubber on the road as compared with cars, need utmost accuracy in torque control to apply power in corners. Four throttle butterflies close to the cylinders do a good job in this respect.
Why Multiple Throttle Bodies?
I think I've posted this response once before in a different thread. Kevin Cameron states in the above referenced article:
"Something on the order of a 10 percent torque boost can be had over a modest range of rpm by choosing the correct intake length. While pipes of this correct length could all be fed from a single “log manifold,” it has always been quite difficult to get
fuel added at a single point to proportion itself equally among the four cylinders at all rpm and throttle angles."
Whether intentional or not, what he's describing here is what GM referred to as Throttle Body Injection (TBI) that was used back in the 1980's, maybe into the early 90"s. While this system was an improvement over carburetors in terms of cold starts (eliminated the choke butterfly) cold engine driveability and throttle response, aggressive corning without one of the cylinder banks suffering from fuel starvation, etc, it didn't hold a candle to port injection (MPFI, sequential port fuel injection, or whatever you want to call it). TBI still had some of the shortcomings as carburetors such as fuel pooling in the intake manifold, among other things. As far as I know, TBI has long been obsolete, although hot rodders can buy aftermarket TBI systems to replace the 4BBL carburetors on their upgraded 60's and 70's muscle cars.
So, much of what Kevin stated in the quoted text above doesn't hold water. While some advantage can be had by using multiple throttle bodies, what happens downstream from the throttle body or bodies is what matters the most, as far as I'm concerned. What I might consider a modest advantage in performance for moderate-paced riders such as me and likely a large percentage of forum members here would be a significant advantage to most road racers and riders who like to take their street bikes to track days on occassion, as well as dedicated "canyon carvers".
That's the way I see it.
EDIT: I did a little research after posting this reply and discovered BMW doesn't buy Kevin Cameron's theory on throttle bodies either. The K1600GTL has a single 50mm throttle body. For those who subscribe to the "more is better" line of thinking in terms of performance, remember that there is no substitute for cc's.