In my experience, some authorities shy away from metal studs to avoid adding additional metalwork into the carriageway. While it seems a trivial amount, I knew several highways maintenance engineers who felt strongly against adding any additional because of the extra work involved during R&R and surfacing schemes. They also disliked having the surface ‘punctured’, as it might become a future point of failure.
Regarding motorcycling, I believe this is commonly and widely felt by those who ride. Even white lining is slippery when wet, and metalwork is thought to be treacherously so. Follow a motorbike and you will often see them riding to avoid manhole covers etc.
Whether this is just perception, or if there is proof, it would be easy to portray the use of metal studs as “anti-motorcyclist”. There are strong cases for using metal studs – the lack of wear and reduced maintenance. This may offset any real or perceived risk. I personally would not use the appearance of them as an argument, regardless of evidence.