Put Another Way...

"We just build the highway. We don't fix your car." 
Randy Bush, INET '96 

Well, IMS attempts to build parts of your car's engine, brakes, and navigational system(s) into the highway
And has a billing model based on where you are going and who/what is in your car 
Which BTW means that the system needs this information too

In short: IMS has as a primary design goal to couple higher-layer services to packet transport 
Note that coupling is one of the primary sources of complexity in dynamical systems (such as the Internet)
See http://www.1-4-5.net/~dmm/talks/SANOGV for a little more on this topic