As a moderator on four subjective sites, this would be extremely helpful on subjective sites. It is a common occurrence for extended discussions to crop up on artistic and subjective sites where the end result is likely to meaningfully improve the answer, but comments is not the right place to do this.
My only option when cleaning up these discussions is to migrate the discussion to chat, but in doing so, the support of various points is lost and it makes consensus harder to reach.
Yes, stars and comment upvotes have different meanings, however one is a superset of the other. Stars ARE used for things that are interesting or that people agree with. Upvotes on comments are used for the same. Stars can be used on other things as well, but that doesn't mean that comment upvotes don't map to them well.
As for the concerns about people with low rep not being able to chat, chat users are already created for migrated comments and the posts are tied to that user, so they'd be able to alter their stars if they wanted to under the normal rules. All involved users are also automatically added to the write access of the chat, so even if they have under 15 rep, they will still be able to talk in that room.