Some authors of conference (or symposium) papers decline to submit a manuscript for various reasons. These include lack of time on their part, lack of visibility of conference proceedings (compared to the journals), etc.
Doesn't that really depend on what you will do with the short manuscript? I personally don't like to have to submit a full proceeding's paper, as that will go into some obscure big book that has indeed very low visibility. One conference I go to insists on a 2 page illustrated abstract, and that put together on a USB stick is a really nice thing to have.
Don't forget that only journals have citation scores, and as long as that is the basic way of judging people, all other expressions are placed lower on the list. On the other hand: if you as a conference set the rules, people will need to go by those rules right?