Apple does not seem to do much usability testing. The reason why people do not use Services is because it was never consistent. Many services, for years, were grayed out, and many of the current services (as can be seen in your screenshot) are buried 3 deep in the menu heirarchal structure. This is also the mistake they made with Automator. It too is buried too deep in the structure.

In other words, it is possible. Word for example has lots of hardcoded custom code.