It's more about raising awareness for a particular charity (
http://www.alsa.org/about-als/what-is-als.html). I certainly don't see how it does any harm, and it has obviously got a lot of attention so far so it is far from pointless. I'd imagine a lot of people are persuaded to donate when they otherwise may not have. Yes, people put far more effort into other charity related activities, but if it's all for a good cause then what's the problem?
The thirsty/clean water argument is totally irrelevant, saving a bucket of water in the UK/US is not going to provide a bucket of water to those who need it elsewhere.