Theoertically, it should be added to any device that you can run commands on (as opposed to "Sensor," which is similar in that it has no attributes or commands--but is intended for devices that just report things back to the hub). This can serve as a "catch all" selector for apps that don't want to use, say,
capability.* (and probably had some use in the SmartThings model Hubitat largely re-implemented, but I don't recall the history there).
You can; there are more apps out there than Rule Machine, and some of them may select for
capability.WindowBlind. But even with RM, this should be usable with a custom attribute selector, as described above, at least once you implement "Actuator" (again, a good idea in general for this kind of device) so there's a capability it implmenents somewhere that the UI does let you select.
(That being said, this certainly could be added to the UI, but unlike some other capabilities, there's not necessarily any obvious choice since the capability defines two attributes...or at least I think it's supposed to and assume there is a copy/paste error in the docs. So, they'd have to make a choice, and if it's not what you were looking for, you'd need to go custom anyway. Not our choice, in any case, but maybe in the future this will change...)