1). Use the "Reply To This" button sparingly )- This is best used when you want to refer to one or two specific points in a member's post. Think of this as a "Quote" function - you want to first quote the specific point(s)you are replying to or are debating/discussing - for focused attention from members. It makes sense therefore to prune the whole Quote (the entire post gets quoted by default) down to those one or two specific points, when you so reply.
This enables a) drawing attention to the main points b) avoids unnecessary waste of space. And as Manish Vacchani mentions, quoting the whole lengthy post(s) add zero additional value. And some posts can be really long! Thank you Manish for being ever vigilant.
2). Use the Quick Reply Text box )- in all other cases.
Use this to make a new comment and/or while replying to a member post in general. Just addressing member by name at the start of comment,serves the purpose while replying to someone in general.
Simple steps like this if adhered to, will enhance communication quality to a great extent.