Doesn't work too well with paper sources. It is unreasonable to expect people to spend large amount of time in a libiary before deciding which bits are and which bits are not sourced particularly if the article is expanded in more than one go or I decide to expand the [[Salisbury and Southampton Canal]] article using Edwin Welch's "The Bankrupt Canal"
There is no way for them to know what bits are sourced correctly without going and finding the source, whether it's cited a dozen times or once. As I've said, incorrectly sourced is far worse than unsourced.