Committees are a means of delegating a few decisions to a smaller group. They're good at taking the time to bring all views on board and arrive at something approaching consensus.
But actual work is a process including many micro-decisions, and slowing each of those down by the committee process dooms a project.
Best practice: delegate the work itself to 1-5 people, then have a committee or assembly critique and possibly approve the final product (document, logo, etc).