You answered your own question above, and also identified why app.net won't interest you. If you're okay with ads, then I think you'd get zero utility from app.net
Are ads in and of themselves really a huge problem? I don't find myself often annoyed by them. Now if there were a systemic change to the service because you didn't have to alter the experience for users to generate ad revenue, then I begin to understand. However if this is the idea, then in what ways the service would be different is exactly what I'm trying to figure out.
Remember, there are two sides to the coin "we offer a better experience without the ads" method. First of all you are going to get a smaller user base. So how much are you going to charge? $5/month? $10/month? You would need to get a pretty massive user base to be able to pay the overhead and attract top engineering talent, so in the end I'm not sure you'd be a whole lot better off.
As a developer, I'm a lot more excited about it. I backed mainly because I was so excited about an API that could've been what Twitter promised. I'm especially excited to see what annotations is going to emerge through it. To explain it in short: Any app can now embed any information in a post. This is big. To give a small example: Say people who allow IFTTT to post their music to app.net. IFTTT can decided to add meta information to it (adding song titles, artists, etc). Now anyone else can easily extract this information.
There is a whole underlying network waiting to be discovered. Anything can now live within in it. What if you wanted to do an instagram type app? Ask users with app.net accounts to log in. When they post a photo, add your own information to it (photo title, photo url, etc).
In the normal app.net (alpha) interface, you won't miss anything. You'll just see someone posting a photo. However, now this new instagram type app can extract this information from a user's feed, using app.net's infrastructure as their social backbone! Any social service can live on app.net's infrastructure now.
IMO, the people who care about ads on social networking have Adblock+ installed. I would even go so far as to throw this (very probably unsourcable statement) and say that most users don't care about ads, period. The one thing that pulls me to social networking is people, not the ads. A social network isn't social or a network if there aren't people I know or care about using it.
Currently, I'm okay in paying $50 to get access to a great community of early adopters (lets admit, it is mostly tech people). I'm not naive to expect it won't change, but once killer apps start popping up on the ecosystem, more users will join which will bring back the value that the older social networks provided.
The highest paying ads will surface to the top over the highest quality status updates.
Twitter is NOT going to compromise the integrity of the entire site for ads. Google haven't. Facebook haven't. Microsoft haven't. And thousands of other sites haven't.
As for Twitter... they've done better than most, so far. But at some point, they'll have more pressure to show revenue. And then... how do they make money, again?