Ah, interesting! I never used del.icio.us myself, but from what I understand, it's fairly similar to Instagram (for example) in how tags work, optimizing for ease of use rather than ease of finding specific content. In my opinion, this is almost certainly the right decision for any platform where "absurdly detailed search" is not job #1, and I'm pretty sure I would have argued the same way as you did.
That said, having seen some of the centralized, intricate tagging systems out there, that let you filter down from Earth to one specific ant in the blink of an eye, that's what I think of when I think of "tagging" that's really effective. YMMV, but I would argue that if you can't type in 10 different tags and get 1 result that's exactly what you're looking for, tags aren't really delivering on their promise.