No, it’s never a good idea to rely on an operation’s side effect; it could be eliminated in the next version!
What I do instead is surprisingly simple. I just call the Count property of Selection. This returns the number of selected items. I call the property with a message box that’s wrapped in a subroutine, which I link to a button on the Actions Menu.
This is all you need:
Sub CountItems()
MsgBox ActiveExplorer.Selection.Count & " items are selected", vbOKOnly, "CountItems() Message"
End Sub
It would be nice if there were an easy way to add a link to this on my shortcut menu.