Split work into whole pipelines

Steve Yegge splits his agents up by job, into planners, coders and mergers. I'm probably wrong, but I still can't understand why you'd do this. No matter what, you always end up with a bottleneck at one of those stages, and wasted work from the ones in front of it.

Why not have a single pipeline that runs end to end, and then run many of them?

(plan -> implement -> merge) * 10

This is my current setup. Every pipeline runs fully to completion, so there's absolutely no inventory sitting on the shelf, ever.

Isn't this the Toyota way?