I haven't seen a pipeline illustrated with a UML sequence diagram yet. I am not saying it is not possible (I think it is) but it would be a lot more complicated and abstract than your [FBD][1] which shows the actual architecture of the simplified processor.


  [1]: http://en.wikipedia.org/wiki/Function_block_diagram

The main problem I see with the sequence diagram is that all the stages happen in parallel and it could become really messy if you would try to illustrate things like forwarding or stalling in it.