I think it's fine if marketers use the service. It's similar to Twitter in that you can choose who you follow. The difference is that if I want Coke in my timeline, I have to follow Coke (or someone cleverly disguised as not Coke) in my follow list.
If companies add their brand to app.net, fine. If people opt-in to follow those brands, also fine. No harm to everyone else.
How do you avoid Marketers disguised as developers?
You will still get the ads no matter what one way or another.