I think they would be potentially very valuable - but as others have said - it only works if you've got connectivity. Unless you've got some sort of satellite connection to the Internet, it may be of no value. It could potentially be helpful in the type of emergency where many people have been ordered to stay indoors (e.g. chemical release or dirty bomb attack). The media would be all over it. But if Tweets were noticed by the right EMS groups, it could give them very helpful info about what is actually happening.

I'd say the main problem is that vast numbers of users tend to trivialize these forms of communication. They Tweet endless gossip. So how would your sincere & desperate emergency message get noticed ... and who would DO something about it??

You could wind up as Robinson Crusoe on Twitter!
HahaHa!!!

Pete2