I'm not at all saying that an editor should depend on others to find sources. I am saying that there is no urgency to add them in each editing session. When you look at the article's history, and see a regular and continuing series of diffs that keep adding material to the article you need to be ready to cut that person some slack.
When I say editing session I don't mean the time between clicking "Edit this page" and clicking "Save", I mean the time between sitting down at the computer and standing up again. If someone wants to save the page multiple times to avoid losing all their work if something goes wrong, that's fine, and they can put the references in at the end. I think it's easier to do it as you go along, but it's a personal thing. If they stand up and walk away from the computer for the night without adding the references, then they've just put an unsourced article on Wikipedia, and that's a bad thing. It's not necessarily worse than no article at all (some would say it is), but it's still a bad thing and shouldn't happen.
When you write an article, you should automatically know what the sources are, because they are where you got the info. If you've written the article without finding sources, then it's written from your memory, which is not reliable and the entire article should probably be deleted. Adding sources is really easy, because you've already done the work before you started writing the article, so there is really no excuse not to add them.