Well you should be ok for now but I wouldn't let it go too long. If there ever were an emergency with the water heater you would have to shut the whole house down. If that valve worked at least you only have to isolate the hot side if there are issues with the tank. Or the valve itself could start leaking. Hard to say really cause we can't predict. If it was house I'd get it fixed, as long as I could afford to, or soon as I can afford it I'd do it. It shouldn't cost you too much to have it replaced, they will have to shut down the whole house to replace it. Probably around 1 hr for actual work. Have you checked your main water shut off in the house? If it doesn't work well you could do them both in one shot. I would get a 1/4 ball valve if they put a new valve in. They are practically maintenance free and won't seize up or at least a lot less likely.