Or, you can have work circles of business partners, press who cover your company, or customers.
I can see this working in other ways. Perhaps a customer support circle in addition to your other customer support forums?
Last, but not least, you can set up circles by interests.
At the moment, Google+ circles aren’t ideal for interest circles. For example, if you were to follow me in a dog-lovers circle, I might only mention pups once every other day or two. It’s my understanding though that Google intends on making it easier to post by topics. I certainly hope they do.