As with Google Adwords (especially PPC ads appearing when I search) this makes sense and, as long as there's a limit to how many ad-tweets appearing at one time, doesn't concern me. After all, if I tweet that I'm headed to Starbucks and get a reminder of their current promotion, that's of potential benefit to me.

Admittedly, the Twitter site itself is far too restrictive to provide me with useful communication data anyway, so I use TweetDeck to track conversations, topics, and mentions. With more real estate, ads would be less cluttering to me.