"Design by committee" is a cliché for poor, soulless design. Consensus among many designers tends to the lowest common denominator, and often lacks a defining central theme.

It's possible to go too far the other way. A single person designing all aspects of a complex solution often leads to uneven maturity within the design. Single designers will concentrate on the parts in their specialty. Very few people are equally good at all the necessary elements.

The benefit of a single designer is to focus the design. Focus helps to prune unnecessary bloat — features that don't serve the primary goal. Focus also results in a unifying theme — a commonality in elements of the design that make a user feel familiar with it, even without having to learn it in detail.

The benefit of multiple designers is to supply more knowledge and experience than one designer can provide.

There's a middle ground that is better than either the lone designer, and certainly better than the committee method.

A Better Way

There should be a single brain in charge of a design, aiming to give it a unifying theme. Certainly in a commercial organization, designs should be formally delegated to designers. In less formal work, design should be someone's baby. But most of the actual grinding work of design is better done in pairs. Not groups, not threes, but pairs.

Having to communicate design ideas to another mind hones the design. If you've ever had to teach difficult concepts, you know that the effort of communicating an idea actually helps one to learn the idea better, to plumb conceptual dark corners, and to check its logic. To present something logically so that a mind can grasp it, you realize whether or not you've chased down the whole logical structure of the idea yourself. The other person can also help catch dead ends, gaps, and problems.

While having to present the ideas to a single (smart!) person is a benefit to working through the idea, the benefit drops off a cliff for each extra person. You only get the benefit of logically organizing idea for presentation once, so that's gone. The only remaining benefit is that the new audience might find some more flaws — at the relatively high cost of keeping another person up to speed. Adding a fourth or more people provides a tiny marginal benefit, while marginal cost of keeping a network of people informed becomes geometrically more costly.

In my experience, adding the right second person to a design effort returns a huge benefit for a reasonable cost. They have to be a designer themselves and relatively equal in skill. Adding a third or more offers only a marginal benefit for a greatly increased cost, and increases the likelihood of deadlocks and a reduction in focus.