If I remember correctly, there used to be a [[Special:Watchlist/clear]] button that would clear one's watchlist without actually presenting the complete list.
Apparently this has been superceded by [[Special:Watchlist/raw]] which allows the user to clear the watchlist by submitting a null string in the text box. In fact /clear now functions as a redirect to /raw.
I think it would be desirable to keep the "clear it in one click" option available:
- To maintain backward compatibility for bot programs which maintain a watchlist and may desire to clear it at any given time using the old method
- To allow users (human or machine) to clear an excessively long watchlist even when loading the actual list would take too long and/or cause the web browser to crash.
By the way, one user claims to have been able to circumvent this by accessing the [[Special:Watchlist/raw]] form using a dummy account with a blank watchlist, then switching back to the main account and sending the form. However, shouldn't this have been prevented due to an invalid edit-token in the form... or is this actually ignored? Might want to double-check.