Yeah, but if it's about the cost alone, why not try IVIG every month for 10 months to take care of the weakness so there will be no crisis, instead of waiting for an inevitable moment you have to give 10 days IVIG in two weeks and even more costs (stay overnight, ventilation, etc)?

Maybe there's some medical reasoning behind it. Like most patients start with prednisone attackwise: hospitalization and start high, after few weeks start tapering down.