I'm got 5000 miles on an 07 b1250 and the original Dunlop 218 rear is down to the wear indicators. It just seems that you should get more out of them than 5000 miles of ridiing on Amish back roads.
I think it's more a question of how hard do you ride. My first B12 had Metzeler's and they wore like stone until I had the bike a few months and I became comfortable. When I started doing the hard accelerations and figured out how to attack the corners they wore fast.
I'm taking Smooth Operator's advice and going with Road Attacks next month. (if it's warm enough to ride.) I hope to get at least 8,000 miles from them but my new B12's primary role will be low speed commuting back and forth to college with the occasional day trip to the Jemez Mountains or Madrid. Maybe even up to the Taos/Red River area if I'm feeling really good one weekend.
On a side note for anybody planning a road trip through New Mexico this summer, these places are "must" visits or you shorted yourself. Just watch the gas gauge out here, stations can be few and far between in certain areas.