What makes no sense, I think, is the notion that a "minority" (the aisle-crossers) could repeatedly defy the actual will of the institution and get away with it. The fact that they never get sanctioned for breaking from what the party supposedly advocates suggests to me that, far from contravening the actual will of the party, the aisle-crossers in fact *represent* the actual will of the party -- considered, once again, as an institution rather then merely a collection of individuals.
Just contemplate how well this works for the likes of Pelosi and Kerry -- they can keep their liberal credentials burnished, to please the folks back home, with fair words and beautiful speeches, but thanks to the aisle-crossers, their opposition can remain purely verbal and cost-free.