Before we begin, yes, K2 does have its own, built-in comment functionality. And yes, there are plugins already built and available for this. The reason I like this solution is twofold:
- K2's comment functionality, while has many useful features (Captcha anti-spam, email notifications, admin approval process), people seem to like using Disqus because it supports logins from multiple accounts (Disqus, Facebook, Google, Twitter). K2's inbuilt commenting feature can be set up to allow comments from anonymous visitors, which opens up your site to comment spam.
- I don't like installing an entire plugin just to accomplish one little thing. While I'm sure the plugin works, you have more customization / control when you handle the actual code. This is similar to the plugins that allow you to add Google Analytics to your site - maybe it's just a pet peeve of mine - but I'd rather edit a (single, easy) file than install a new extension, which is a potential security risk.