Super-sweeper, Dyson recommends that, for allergy sufferers, the bin should be encased in a bag and then emptied. But if you're emptying a bagless bin into a bag, then you might as well have a bagged vacuum.
Plastic bags are very easy to obtain, whereas with a bagged vacuum, you have to find the correct brand and type of bag your vacuum takes. This can get expensive.
Marcusprit, yes, that is true for all bagless vacuums. Many vacuum manufacturers make it easy to dismantle the dust bin for cleaning the shroud, cones etc. I empty the bin after every use, even when it's not full.