You get a nice box in the back or whatever but it's not really THAT much bigger than a box you could fit on any old regular bicycle
You can plop like two children there but then you can do that with Long Johns and 3-wheelers and such, too and ALSO you have the fuckoff-big box. They don't really seem all that shorter than comparable versions either.
Anyone got one or know why someone got one?
My buddy has a longtail Surly that he uses to run shuttle for kayak trips he takes with his daughters.
Because the long tail offers, essentially, unlimited clearance out the back, he can sling hardshell kayaks on the side of the bike (in harness pannier bags) and just have them stick out the end while his girls ride on the plank. Don't really think that specific situation would be workable with a bakfiets.