All you're going to accomplish is encourage people to stop quoting entirely by making it too much work. Not only is it too much work to quote manually, it is also going to be difficult to determine who someone is replying to if when they don't include a quote in their post causing everyone to have to click and scroll back through the thread to figure what the hell the last poster is talking about.

It would be like if I suddenly out of the blue made a remark to you in person about a conversation we had two weeks ago that you no longer remember but I make no reference to that conversation to jog your memory when I make the remark. With no point of reference, you're going to stand there and stare at me like I'm speaking to you in a foreign language. In a very long thread with no quoting, it's going to be too much work to try to guess which post is being replied to.