Consider any forum visitor. How many people are selecting text in a post to simply quote it in a reply versus how many are selecting text in a post to multiple quote?
I would say the majority of people simply want to quote the text selected. Now, what is happening is that I have to click three times and wait for an overlay in between simply because I want to quote a single line of text in my reply. This is too much of a hindrance for the end user.
If someone selects a part of post and clicks on quote, it should directly go into the reply box. Next if someone does it again, the previous and this second one can both be added to the multi-quote dialog box. This way the majority of users are not inconvenienced.