While I agree that we should utilize the available technology to the utmost, ancestry.com explicitly disallows copying and reposting of their digitized documents to other websites in their user agreement. If ancestry.com is the only place where a digitized image of a document can be found it is absolutely legitimate to use it as a source without posting a copy, when the user agreement prohibits such a posting. I do agree with Jillaine, though, that such a quote cannot just be a link to ancestry but should also give sufficient detail about the source.
I am currently working mostly with digitized church books as sources that are posted on a free website. I still provide the name of the archive, the parish, book number, page and image information besides the link to the image. Who knows if this website will always be free?