how to re-enable services in an app (Microsoft Remote Desktop in particular) I am fairly confident I could use the service below in MRD before. I am not sure when I lost them, maybe it is related to the Yosemite upgrade (I noticed this on 10.10.0) or some recent MRD update (8.0.9 from Mac App Store). In any case, the Services menu (from the app's menu on the menu bar) lists the services I want to use, but all are greyed out an inaccessible. Why is that? Can one undo this?

I checked the services under the Keyboard preference pane, and they are enabled. They also work in other apps.

It is not related to any restriction on any specific RDP connection, even without connecting, the services are unavailable.

The services is simply an automator workflow left under ~/Library/Services, with running AppleScript:

on run
    repeat with theItem in (the clipboard)
        tell application "System Events" to keystroke theItem
        delay 0.01
    end repeat
end run
Some (perhaps all) services are context specific. They will only be offered/enabled when they can be used.

For example, a service that is meant to act on a piece of text will only offer itself when text is selected.

You can try this yourself in Safari. Select a piece of text in Safari and check the Services menu, you should see numerous options including "Add to iTunes as a spoken track" and "Make New Sticky Note". However if you visit the Services menu in Safari when nothing is selected you will see "No Services Apply".