There are two questions there: what's wrong with email for Twitter and what's wrong with email for users. The first is easy: they don't control it. Twitter han

There are two questions there: what's wrong with email for Twitter and what's wrong with email for users. The first is easy: they don't control it. Twitter handles are already a broad system of public identities and Twitter wants to double down on that. The second is harder. The value proposition—if any—comes from letting people stay within a single service. It lowers the barrier to starting a conversation on Twitter since you don't need the song and dance of exchanging emails first. It also helps people keep their public identity consistent, especially for those who (for whatever reason) treat their email address as semi-private. Is this enough to make people use Twitter messages more? I think so, especially on the first point. If you make something easier, people are more likely to use it. I doubt it will replace email to any great extent, but rather lead to out-of-band conversations that wouldn't have happened otherwise. We'll have to wait and see.