I installed a fuel valve in mine a few years ago, while replacing my ruined carb. I thought I didn't really need to exercise the generator every month, if I just used the drain screw to drain the carb before storage, any time I used it. But after it sat unused for a couple of years, I discovered that it wouldn't start when I needed it during a power outage at a Nashville park.
I think the real reason you need a shutoff valve is that, after you think you've drained the carb, fuel slowly leaks through the fuel pump and fills it back up again, anyway.
By the way, after turning off the valve and running the generator until it dies, you should still use the drain screw to drain the last bit of fuel from the carb bowl.