I think things are getting a bit muddled. The ‘reply as linked topic’ would be good if you eg. read a poll in the Sandbox and wanted to have a conversation about eating meat. It’s a way of linking back to the post that inspired you, without taking the original topic off-topic. 
Though the Lounge is also a good place to have a sidebar too. I think it would depend on how big a conversation you’re looking for.