I'm actually surprised to see you ask why it's necessary. Of COURSE it's necessary. We live in a world of bots and automated registration that only leads to spam, spam, and more spam. If they didn't require captcha for all user-agents, then someone could just spoof the user-agent header to pretend like they were an iPhone and make 10,000 accounts.
Hey, great post. So many twitter clients and tools out there it's hard to keep it straight. I'd like to point out that you forgot us overhere at Sociagami. Update statuses to multiple services in one click, including Twitter.