Well, Today I was probably one of the ones you are referring to. I often do trim down quotes. But on the one I did this evening, I felt it important to note the text I was referring to since it was a very busy thread and my whole point was that I felt the entire post presented excellent ideas and was well stated. In highly active threads where many people are making points in rapid fire fashion and it often rolls over to a new page view, I find it beneficial to see the related text close to the current one I'm reading.
As to the readability, this is nowhere near the problem in this forum that it is in Yahoo groups where there is no formating. In this forum, when reading it in the browser window, the formating makes it very easy to follow and discern quotes from fresh text. Now if someone is trying to just follow a thread by reading the email notifications, I can see your point. The lack of formating makes it difficult to follow just like with a yahoo group. And that is even more true when abreviated multiple quotes are used.