Twitter Micro Blogging versus Mobile Blogging with WordPress

Twitter has got the blogosphere acting crazy. I’m not sure why. I can see why it’s interesting, but if someone has a blog and is able to post via a mobile device, such as a mobile phone, then why would you need Twitter? Why would you need a Twitter ‘micro blog’ if you already have a blog. Surely the only difference between Twitter and WordPress is that Twitter encourages short posts, whereas WordPress, by definition, encourages lengthier conversation.

Both platforms allow remote posting.
Both platforms are free.
Both platforms are stable.

However, as the Twitter service develops into a more mature service they will have to start charging for posting via SMS or Txt. When that happens, why would you want to use Twitter, especially if you already have a blog?