Logo is important if you want to build a brand, or if you want to work with someone who wants to build a brand. Its basically your company name in image form. Even if you are still a very small business, having a logo that your clients recognize is essential for maintaining a relationship. Not everyone needs one, but if you are working as a group, or remaining faceless, the logo gives you an identity.
I like to think of it as similar to a wax seal on letters. A good logo will tell who you are. More importantly, a bad logo will give off a poor image. I think that not having a bad logo is more important than having a logo, because it indicates that you don't do well at what you do to potential clients.