Sometimes there is no need for a reply to have the original email included, it may be that you are responding to a particular point, or need to to draw attention to a specific paragraph. Using Gmail you can now quote text you have highlighted.

Simply select the text you wish to include in your reply by left clicking and dragging your mouse then choose reply or reply to all as normal. Your response will only include the chosen text and not the whole email. If the email you are replying to was part of a conversation, it will remain in the conversation, but that email won’t have the whole email string quoted – a useful feature.