Yes, since we're keeping the one-off fee option, this would be the situation with most modules. Some products may not be suitable for subscriptions at all (like, say, installation service) and some would only be suitable for subscriptions (like, say, hosting or administration service).
I don't see why not. Now we have "services" category in Market, but they are all either priced as one-off or just advertise here with payment processing on vendor's site. I think vendors should be able to offer things like, say, RMS hosting or "site administration" service with monthly subscription.