I find it hard sometimes to continue my development momentum when my applications user interface is not developed nicely.
On the other hand, if I have a UI then has all the elements of the application, I find that I am much more excited about the project. I think the main motivation is that I have a clear picture of where I'm headed, and I already have the end goal in plain view. Having the end result infront of me allows me to reverse engineer how I want to go about designing the application also.
Another benifit to developing with the UI designed, is that you are forced to really think through how your application is going to work. Reason being is that you won't be able to even comeup with a mockup of your design unless and until you have a good understanding of your sites functionality.
Downside? Unless your a designer, your design bill might be a little higher than normal since there are bound to be a few tweaks to the layout unless you really hammered the design phase (since the design phase is the 1st step in this backwards approach).
Interestingly enough, the people at 37signals have a little ebook out that goes over this approach in a little more detail: https://gettingreal.37signals.com/