The only people I can see using this are advertisers. They would be willing to spend money to get their stuff up in front of an audience. And why would I want an iPhone app that just has tweet-style advertisements?

Imagine it like billboards on the highway. They cost money, lots of money. But it's not like this paid aspect results in good content. Paid placements always attract people who have something to gain from the placement.

And expecting your users to spend money and effort in order to clean up spam is asking a bit much in my opinion.