While this makes sense for Steemit or social blogging frontends, maybe Steemit Inc is looking at an opportunity for a Disqus-type competitor where the Steem blockchain would host comments on websites of various kinds.

This was the reasoning from their initial proposal from @steemitblog:

Independent Comment Reward Pool

Comments have a very different level of visibility and therefore get considerably fewer votes. In the past month only 1% of rewards were paid to commenters. Due to the nature of the N^2 reward curve it means comments are not competing against other comments, but against the top bloggers.

We feel that engaging more people in discussion and encouraging higher quality comments will make the platform more desirable. While relatively few people want to blog, many more are interested in commenting.

If all comments only have to compete against other comments, then more users can participate and comments can collectively garner a larger percentage of the reward pool. We are proposing that comments be allocated 38% (golden ratio) of the current reward pool and that comments be rewarded on a N log (N) curve with some to-be-determined modifications. This should work to allocate more rewards to those who contribute to discussions and drive community engagement.

To me, it simply appears that the focus is driving engagement on Steemit.com. And I would imagine that if they wanted to create a separate pool to be used on different sites strictly for commenting on non-STEEM/Steemit content, then they would probably need to create a side-chain for that, wouldn't they?