I get the idea here, but I find planning very useful as a phase when I'm doing things I am not intimately familiar with (e.g. AppKit programming).
That's when I need to learn about implied patterns, do's and don'ts; not from theory but in the context of my own project. Unfortunately, the model tends to keep implicit knowledge implicit. But I can ask during planning.
The feature did get less useful over time when the model started babbling in newspeak more and more. When it threw a thousand words at me even in concise mode.
So I don't want to let the toolmakers off the hook here. There's a lot to win that would make plan mode much much better without changing plan mode itself.