I personally hate when Email threads get to be very long. It is easy enough to remove unnecessary text from your email replys in Gmail or Google Apps. You need to enable this via Google’s Labs feature. You can do so by:
Going to settings, go to labs, locate and enable:
Quote selected text
Now before you hit reply, highlight the block of text you want quoted and then hit reply. This will only include the highlighted text in your reply! Thanks Google!