Sometimes I hit balls at a covered outdoor facility--into a net. It's an interesting setup, but if there was no net the ball would be launching over la Seine into some high rises.
Anyway, I find it very worthwhile compared to not doing anything. Sometimes when you are working on something, not worrying about ball flight can be a good thing. Practice swings in the back yard are ok when you're working on a grip change, right? You either make contact on the sweet spot or you don't.
Doing drills is good for this type of setup--you're not that interested in where the ball is going in some cases anyway. If you can get a good mirror, you can check all sorts of things. You can also put a line/marker on your net/tarp and check your start direction (assuming you have some ball flight time). With all the recent discussion on zeroing your path and learning to hit "straight" shots, this can give you some data. Work on trajectory if you like.
If you are a good player, your fundamentals don't change day-to-day. It's the feel for the clubhead, tempo, and balance that you can be working on indoors. Wouldn't do it all this way, but you do what you can. Cheers.