I have been a BizSpark member with two different companies, as well as the Empower program prior to that. I have also worked for companies that are certified resellers. Although I am currently not using the MS stack, I have made my living in that stack for over 15 years.
BizSpark is most definitely something where you decide how involved you want to be and benefits will come based on that. Signing up gets you limited time licenses and you can choose to be listed in a directory. I don't know who actually reads this directory.
However, MS makes available partners and help and advisors on just about everything, if you want to take advantage. It will still, of course require effort on your part to follow up on the advice.
The program is not (nor could it be) designed such that you sign up and they do marketing for you. There would no way to know what direction to take.
If you are selling software that requires installation inside an organization, including purchasing MS licenses for servers, SQL, etc., MS can provide lots of support to move those products in the form of marketing materials, training and demo products. Their interest here is enabling you to be their affiliate program and move their software for them.
If you are using the program to work on web-based software, MS will help you technically make it happen, but the marketing side is pretty thin. In this case, their interest lies more in keeping developers on the platform instead of leaking to other alternatives.