It depends on the application, but the 2 vast graphics cards that make up a sizable part of the Pro's power draw will go to waste completely in a typical server application. The setup could be interesting for compute tasks or specialized render farms, but then again similar performance could be had with much cheaper components.

I don't think there are lots of applications where the use of a Mac is actually a financially sound decision in this scenario.

I agree with this. There aren't enough applications yet that even touch what the Mac Pro can do, and when they do exist, limiting them by network bandwidth through colocation seems like a dumb move. Give me a farm of Mac Minis any day.