It works for Valve, because everyone in those meetings are pitching ideas in the areas they are capable of and willing to do ("I'm the artist and I think it would both look cool and be visually coherent if everyone had cartoony Silhouette! I can totally do that!"). Even then, the project lead still needs to sign off on it...and are often the only ones at the meeting anyway:
The initial Cabal group consisted of three engineers, a level designer, a writer, and an animator. This represented all the major groups at Valve and all aspects of the project and was initially weighted towards people with the most product experience (though not necessarily game experience). The Cabal consisted only of people that had actual shipping components in the game; there were no dedicated designers. Every member of the Cabal was someone with the responsibility of actually doing the work that their design specified, or at least had the ability to do it if need be.
Internally, once the success of the Cabal process was obvious, mini-Cabals were formed to come up with answers to a variety of design problems. These mini-Cabals would typically include people most effected by the decision, as well as try to include people completely outside the problem being addressed in order to keep a fresh perspective on things. We also kept membership in the initial Cabal somewhat flexible and we quickly started to rotate people through the process every month or so, always including a few people from the last time, and always making sure we had a cross section of the company.
The key words here are "responsibility". Those people with it always get the last say.