I don't see the billing and activation systems merging until such time as they merge the spectrum under one system. Financially I think they see this as the wrong move because it would lower the amount of income they have coming in. Clearly a merged billing and activation system would be best for consumers, but at this point not best for Sirius XM's bottom line. That is the reality of it rather we like it or not!
I think you are right but for the wrong reason. Its probably not loss of revenue that they are concerned about but the expense of developing a unified system. I don't think the number of dual subs, given the unification of content, is significant anymore.