I've written succesful applications without any spec, just talking with users, writting things down myself, showing them prototypes, repeating. In some cases users are not able to produce a spec and even if the do they don't always really "know what they want", that's why prototyping is so important.
I've worked on projects that didn't have usable spec, and monthly meetings had client saying "oh, that's not what I've meant, can you redo this like that?".
Oh, and users never know what they want, even when they think they do. What they know is the problems they have and the job they're trying to do or want to do. It's the job of the software designer (both spec writer and programmer if they're different) to figure out what the software has to do to give the users what they need.