Why? You can simply share the URL to the post. Every Discourse post and topic has OpenGraph / oEmbed tags so it will display appropriately, with an automatic excerpt. Try pasting a link to a Discourse post in Facebook or Twitter to see what I mean. Like so
This would also make quoting more confusing, and elevate the rare use case – “I need to share just this third paragraph of the post” – above the simple use case – “I need to share this post”.
At best plugin material.