Why there is not a possibility to apply to one content type another content type as its comment type in Drupal core?
I think it maybe possible to create content type to be a comment on another type and reference it to main node content type. So when we referenced comment content type to a main node content type, there are automatic created fields for comment standard information.
What profit do we have if we did it?
The first - we will not have a problem with search permissions on site. like in http://drupal.org/node/680992
Second, we could enable user permissions to create comments on one content type node and disable permissions to add comments on another content type node just because we can do these comments type by different content types.
Moreover, this possibility allows us to do all that we can with node reference but much more simple.
And of course, we could define a very complex comment type.
Comments
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Because it's an edge case and therefore not suitable for core-- thus the existence of the nodecomment module.