The reason, I believe, that FTM does not encourage citing specific User trees as sources is their own brand identity. Ancestry.com wishes almost all sources to be identified as sourced from them, even when users locate the citation and include it in their tree. The default template for most databases on Ancestry.com cites Ancestry.com as author, publisher, and repository - even when the database is nothing more than scans of a 200 year old book. That is plagiarism in my view. I tend in my own tree to look through a User tree to its cited sources and record both. Most User trees are nothing more than a transitory compilation of secondary sources with little quality control and no permanent record. So, there is a question in my mind as to whether User trees rise to the level of a source.