I feel sharing makes more sense for posts than for comments, since comments usually need context to be valuable. There are very few comments I find that I feel deserve to show up context-less in the chatter feed of those following me. So maybe two buttons isn't a bad idea; one for sharing the comment with your followers, and one just for affecting the order of comments in the comment section.
On the other hand, you can often find interesting threads through those shared comments. The 'parent' link is there for this exact reason. To use my own browsing habits as an example: I'm filtering almost every tag directly related to politics (and quite a few other topics, like brands). They are oversaturated and mostly orbit around same thing ad nauseam. But despite that, the responses those threads sometimes generate can range from hilarious to insightful (sometimes both) and tend to be of better quality than the OP.