Social media networks are becoming an increasingly important communication medium between you and your customers. Like domain names, customers expect that twitter.com/YourBrand leads to your site. They also expect that twitter.com/YourRealName is a direct link to you. A competitor having control of your name on social media sites means they have a direct communication channel to your customers.
Even if you have a well-established brand, it's important to protect the brand on social media networks. As you can imagine, the cost of recovering the brand after someone else already has it (if it's even possible) will be far more expensive than pro-actively registering it now.