Firstly, this is just to point something out that people doing it might not have thought of, no intention of offending anyone so don't get in a huff
There are a lot of people that continually quote everyone that they are responding to. There is no need to quote someone that you are responding to if you respond in the same thread. Quoting is something best used when you want to respond to a specific part of someone else's post. Even then, if you are responding shortly after the person you are replying to has posted then there is no need to quote, everyone who reads the thread has already read what the other person has said.
Each time you quote someone else's whole post you're making reading the read more difficult for others. Instead of reading your response they first need to get past the bit you quote.
OK, I feel better saying that, mock me all you like now
ps. Some examples of how to or how not to do it below with the purpose of assisting with the explanation of what it is I am talking about.