I started a blog primarily to promote my start-up company (it is under a ".blog" sub-domain of my start-up site) by helping the page rank of the site and by driving more traffic to the site. It is still in its early stages and I only have 2 written posts up, but in the future I aim to get more authors to contribute on topics that relate to my start-up. I also want it to personalize our start-up company since we are only 2 hard working hackers trying to make a go of it, and I think that will appeal to our customers.
I started working on putting Google Ad-Sense ads on my site to see if getting some extra revenue from ads would help pay for server costs of the blog and the main site. Is there really a huge backlash against start-up blogs that show ads?
I wouldn't be afraid to add one or two small slots in the sidebar. As long as it doesn't become your main source of revenue I wouldn't be put off by it at all.