Don't really see the relevance of an about us page to a site's overall SEO. It would be good to have based on what your website is up to. ex: If its an estore then visitors would want to read up about the site, where you are located and other stuff.
I don't see any reason why it would be essential, to be honest. With that said, it can be a good place to have an article with some good specific keywords linked to your niche and/or your area. It can also be a good place to have a few links which can take visitors to other places on your site, which will reduce your bounce rate. You could also use it to do a little bit of pre-selling so that when people come through to the main site they are in more of a mind to spend money - assuming that you have a store. As I say, there is no reason it's essential, but at the same time it's a good opportunity to tell people how awesome you are and why you are awesome. Though I wouldn't put it in those words...
Why don't you have it? I see more value of having th epage if only to kw stuff for seo reasons. Suffering seo results by abstaining perhaps.
I don't think it's really necessary. I would only add it on a website where it would be useful to my visitors. For example, if you're building a website for a family business that's been around for 100 years, I'd definitely have one. If you are building a porn site, then not so much required. It is only one small page, so I doubt your onsite SEO would suffer that much unless you have very few pages.
