Evolution makes less use of it. Evolution uses the pair in conjunction to find components available on the system. If there are no components which can display something in Evolution, it can look for programs to perform the same function. This is nice because you can achieve consistency between Evolution and Nautilus, apparently. I imagine they were thinking of things which you are happy to run on a local machine but which you wouldn't run across the network or something.
Evolution has a split between 'model' and 'view'.
The model contains data. The view contains how to display it. The wombat back-end contains information and data so is an example of 'model'. That's because they're not sure what goes in which bit and how best to draw the split. Evolution hackers want to handle many many folders and huge folders, and they want to handle lots of data over the net.