Sure, a captcha of "lI0Ol1o" would would probably be unreadable to a computer ... but it would be to a human too.
We're quickly approaching the point that image recognition is getting as good at solving image captchas as humans are, and when we do, we'll need to find some other way to do it.
Which is exactly why they added those. Premium accounts do not require the user to enter a captcha for every download. So every user who was annoyed by the "cats captcha" was a potential customer.