I see on the Motorcycle Superstore site that the BMW R12RT calls for a 'special' B rated tire, while that is not so for the Michelin tire fitment guide. So I called Michelin and spoke to Ken, customer service manager, and got some information:
The special tires are for higher loads. They do NOT increase the weight rating of the tire, but they add an extra ply to stiffen the tire for lateral turning loads.
This is recommended for two up or heavily loaded touring bikes.
So, if you are like me and don't go two up, and don't carry a lot of weight, you can go with the cheaper tire.