The strongest work of my career to date came about on a product released before it was finished. Features were incomplete, others un-styled, many missing altogether.

Why ship it early? Motivation.

Nothing is more motivating than thousands of people using your incomplete product. Bugs are fixed immediately. Designs that exist in Photoshop are live in two days, and their weaknesses will be pointed out immediately. Un-styled placeholder markup built by an engineer forces a designer to stop dragging his feet and design a real solution.