PLUS:Less hop debris to deal with. Smooth bitterness. Easy to know exactly what the bitterness level will be.
MINUS: Can't think of any.
I have been using extract for bittering for my last few batches. I have enough extract to last me for a lot of beer, so I am committed to using it.
As to Davids comment - don't understand the "complexity" thing when it relates to bitterness. To me it is either smooth, in the background and of little notice, harsh or puckeringly beyond pleasant. With extract I am the master of my embittered domain!