You need water for many different things during a disaster than just drinking. It's probably the most critical resource you need for a disaster. During a major incident where your power is down and there's no other clean water available, you'll need clean water for cooking and hygiene purposes. Since the majority of cities in First World countries obtain their water through grid power, a long term power outage will leave towers unfilled rapidly and then you're out of luck.
For short term disasters, like a power outage for just a few days, you'll be fine drinking whatever you want. Beer is mostly water, so you can get by on it short term. The problem with beer is alcohol reduces the anti-diuretic hormone (ADH) production in your body, making you urinate up to three times more urine than normal. When you're urinating a lot more, you need to drink more fluids to replace what you lost hence if you only have beer to rely on it's only going to make it worse for you. This is why soda pop isn't a great idea either - caffeine has a similar affect on ADH.
Long term though (and they do happen - after Hurricane Katrina it was more than just a few days for many folks to get what they needed) water is the far better option as there are just more uses for it.