If the desired result is to sell more product, tell me how knowing your product helps me achieve that, when you are asking me to sell that product via advertising or SEO. See it all boils down to the consumer. What you are missing when you want your agency to be an expert in your product or service is the unbiased knowledge of the market place.
An ad agency needs to be able to tell you with out “contamination” that there is something better our there for you. Think broadcast news here, you don't want our judgement clouded or biased because we are required to punch your clock every day. In advertising it should not be about you, it should be a consumer driven focus that will lead to increased business. So please consider this when you look to an ad agency for help.