It was done that way because @blocktrades wanted the HBD to be received right away, as you say to sell into pumps. I would have been perfectly happy to have it work the same as the other conversions. Yes it would be less effective short term on pumps but I think it would have been sufficiently effective over time and simpler. In reality, we have no way to know if this is correct.

Anyway, the way it works is the HIVE you "convert" is actually posted as collateral for a loan of HBD. The amount of the loan is determined using the MINIMUM feed price with a 2:1 collateral ratio (i.e. you get half as much HBD). Then after 3.5 days the conversion actually happens at the MEDIAN feed price, converting whatever portion of the HIVE collateral is needed to repay the loan, and you get a refund for the rest. With 5% fee deducted of course.