We humans are visual creatures by nature. Over 80% of our outside sensory comes from our eyes and thus it is natural for us to make decisions based on looks. It is the same as looking at a person and deciding whether you like that person or not, even if the person is a bad person deep down inside.
We have a tendency to go for the more visual appealing looks. Thus the importance of a good looking logo.
A logo should be easy to remember, unique, simple and effective.
Every brand, website or just about any product on the market should have a logo designed for it. Even this forum has a logo designed.
Every IM-er on the market should have a logo on his web site. It promotes trust and a feeling of a good and respected business.
Just ask your selves: How many times have you tried to remember a website that you've been to and just couldn't because you had nothing to remember it by? Well a logo eliminates that problem.
It's the little things that matter.