Well, my few cents here because i also thought about this months ago.
I understand both points but from my long commenting experience and using tools like IntenseDebate or Disqus i noticed that the person who quotes someone else does want to stay to 95% inside the related discussion.
Sure there are some people from time to time who quote several comments at once but in this case they use mostly the quote tag instead of using the quote button.
On the other hand i understand totally what Daniel says:
I'm quoting you, I'm not replying to your comment. That are 2 different tasks.
I have on my old site more than 5600 comments and some of my members asked me the same.
I gave them mostly the same answer like Daniel above and told them to "reply" and then to use the "quote" tag if they want to stay inside the actual discussion.
Sometimes i was really happy that some people quoted and that a new un-nested discussion started because the comments were nested, nested, nested and nested and after 120 replies we all had only a small frame to reply
(which is normal).
All in all i would do it like this way:
Add 2 buttons:
- Quote as Reply
- Quote as New
This would solve it and all would be happy.
Only my few cents and a suggestion.
greets!
Marin