In my own mind, I cannot rationalize it. The ASN program is 18 months. Once you graduate and pass boards, you can nurse. At that point you can immediately begin taking the classes needed for BSN. But from what I understand, if you are enrolled in the BSN program from the start, you cannot nurse until you've graduated the program.
Other than not wanting to take classes while working, I can't see why anyone would choose to take the long route. I'm sure some have their reasons, though, which is why I'm inquiring.