The average customer desires a personal connection with companies they shop at. This largely explains the revolt against WalMart. They are seen as a massive corporation that is only in the business of making a profit. If customers didn't care about the personal shopping experience, small towns would welcome stores like WalMart who sell products at much cheaper prices than smaller Mom & Pop stores.
Twitter is the personal connection tool that large corporations have been waiting for. They can show that they actually care. They can reach out directly to the customer. Comcast is a great example of putting personality behind a corporation.
What is stopping most companies from embracing customers and starting up a dialogue with them?
Fear.
Companies fear a backlash. They fear one poorly worded tweet being retweeted and causing an uproar. The latest example with Motrin is not reason to avoid social media, but more so a reason why you need to know your audience. Just because one company in your industry fails, doesn't mean your company will. The same applies to social media. Just because one company screws up, doesn't mean yours will.
The day that a company loses control of its brand online, is the day the company will begin to build value in the eye of the consumer.
I welcome any company that joins Twitter, as long as they use it to start conversations and not shout advertisements.