It doesn't work well in practice.

Try it yourself, use a big model like Opus or Sol to implement everything by first making a plan using plan mode.

Then try distributing the task to a cheaper models like Luna Max or Gemini Flash 3.8.

During planning, the big model already reads the relevant files in context, while giving a smaller model a slice of work itself requires the big model to reason about the task distribution, review, etc.

So do you really save on tokens?