For me, it was a long time before I forged first-name-basis relationships with schedulers. There's attrition among schedulers, shoppers, and clients.
I determine MSC preferences based on communication and expectation, coupled with the client/reimbursement being worthwhile. Based on that, if I've had a mutually wonderful relationship with an MSC, great. If the MSC loses clients of value to me, I don't deactivate myself, but it takes a backseat in my lineup.
MysteriousLurker, I have shopped with, and liked Bestmark for years, and agree with you, wish they had more variety.