For me, it is a killer feature -- any system where dependencies are maintained manually inevitably gets out of sync with the code. Some systems have special compiler hacks (like gcc's -MM flag), but you still have to find the magic option for each program you use, and link them into your compiler. That's also very hard to do in dynamic languages, where you don't know what libraries you have loaded until you've run the program.

I do use auto-generated/-detected deps for build time dependency graphs, but I didn't like how tup implements it.