Your intuition is correct, getting most-bookmarked-link in a tagset is an awesome way to whittle down solutions to find really common links that are tagged. I recently needed a quick solution for an HTML form that I needed to be emailed with PHP for an old client that had been using an offsite service that broke.
I went to delicious.com/tag/form+php+email and found a lot of bookmarks tagged. I tried going to delicious.com/popular/form+php+email but it just truncates the single, first tag.
Its like Stack Exchange says--the best result should float to the top! Why didn't Yahoo think of that? Why can't we sort all bookmarked items? Is it some sort of caching concern?