I agree with the others here. The moderators have also been given the ability to delete all comments on a particular question or answer quickly, and when comments start to outweigh the value of a question or answer we often delete them with a note stating this.
To summarize the sections below:
- Comments is a useful moderator tool
- Comments are disposable
- Comments can be invalidated
The comments is there to provide awareness of shortcomings of an answer when it does not constitute someone posting a complete new answer. I have to admit it has become a useful moderator tool on the main sites since it helps us quickly identify problems or areas we need to address.
It is also less likely that we will delete an answer rather then a comment. I would suggest you spend a little bit more time on the site and you will soon see why comments have such a limited scope. There was a stage where comments weren't allowed at all.
The OP of the question or answer can edit these at any time, which can quickly invalidate comments, forcing them to be removed. So you may add something significant in a comment, and the OP decides to delete it later, or change the whole answer to be more effective. This has happened in the past.
"Twitter-like" is a very elegant explanation for why this is such a terrible design.
To answer this comment. What you consider a terrible design has taken more then a few months to tweak and work at to get right. SO was not slapped together overnight, and Jeff and the team works extremely hard to implement useful features. I would further suggest that you spend some time going through the past feature request, and you will see how much work has actually gone into the commenting system.