Just the implementation...The customer has two controllers in master/master. As the user database does not replicate between them, then you have to teach help desk, secretaries, etc. to create users on the right controller. Also, who cleans all unused users? Sometime what you want is just give a 1 day voucher but some other time you may want to give one week (i.e: training course 1 week long). In this case, guest management guets more complicated. Of course, still doable using the native interface...
Mo