do you mean one with a trailer kinda deal to it? or a friction drive kit?
if you mean a trailer style one like the emoticon i can only think of one positive aspect to it, detach it and you're right back to a normal bicycle. negatives would be its big clunky and awkward, probably impossible to get registration for, harder to do while offering no real advantages from what i see and i don't see it handling as well as a in frame chain drive or rear mounted friction drive kit.
but thats at a glance with no experience of any kind with the trailer type of motor. so maybe i'm wrong