I left my Mazda Protege unused in my garage for over a year, and the fuel filter was clogged when I went to start it again. It started fine, but would not rev and was basically undriveable. I'm sure it wasn't great for the fuel injectors, too. Also, I once bought a motorcycle with 10 year old gas in the tank, and the bike wouldn't run at all. I could hear a cylinder occasionally fire, but the gas was totally unusable. I poured it into a metal bucket and lit it on fire. The carbs were very badly gummed up and clogged, too, and had to be torn apart and soaked in carb cleaner.
If you use STA-BIL you can store gas for over a year without issue. Just make sure you run the engine for a few minutes to circulate the STA-BIL through the car. Once gas goes slightly bad, you can still use it if you put a little (1 gallon) in an (almost) full tank of fresh gas. The stuff I pulled out of the motorcycle was much darker than normal, and smelled so bad that I didn't even consider using it in an engine.
If you're planning to store a vehicle for a very long time, it makes sense to spend the time to totally drain the fuel system. It's cheaper than replacing the fuel system, later on, due to dried up, gummy gas deposits.
Here is some info I found on the STA-BIL website:
Q: How long will STA-BIL Fuel Stabilizer keep fuel fresh?
A: For 12 months when mixed into fresh gasoline. Doubling the dosage will keep fuel fresh for up to 2 years.