For myself, I don't have any use for it. I'm busy enough with things in my own life and don't have time for yet another channel for distractions. I have face book, but only log on maybe once a month. For professional connections, I use linkedIn, where discussions are meaningful and networking is very productive. Everyone behaves because your reputation could be damaged if you engage in certain types of behavior.
You have to keep in mind there are producers of content and consumers of that content. For a business, it makes sense to use twitter because its a fairly cheap way to reach more customers; no need to manage or lease a server farm for electronic correspondence. If it generates revenue, great. It can also serve as a sales loss lead for future generated revenue. As for movie/sports stars and their ilk, I think they have the free time for the mundane communication.
Many people I hear of using twitter to "find good deals" end up buying things they would not have bought otherwise; meaning they are actually wasting money, not saving it.