I agree our tree should be accurate and sourced. I am very big on sourcing. I am also against using other online user trees because they are very unreliable.
Now, the monkey wrench.
First, that is a strong statement and although true, we are talking about throwing it at new members who have not yet learned how to operate WikiTree yet. That would be enough to scare many away. I work with new members all the time and many of them are nervous and have a lot of questions. The way that is worded would definitely be intimidating to a lot of new members.
Second, while I really do not like user trees being used as a source, what will you do about GEDCOM's? We allow and encourage members to join and upload them. What is a GEDCOM but a user tree? They, upload an entire user tree to our system, and 9 out of 10 times there are no sources. I believe that affects the accuracy of our tree and we allow Family members to do that. They have not even signed the Honor Code. So, would this not be something we should consider too?