An obvious benefit to plan mode is that your model tells you what it's planning to do before it does it.
Removing edit tool from your model is like lobotomizing your engineer.
Here's the secret: Just building the thing is the modern "plan mode". You don't have to keep what it made. But a completed, verified, and tested output is always more valuable to my planning process than a hallucinated sloppy guess.
I do not use an explicit "plan mode" that tries to limit what the LLM can do. But I do tell it to explore and research and produce an artifact of what needs to happen to finish a large ticket and then use that as input to the actual implementation. I do have it even write small experiments to verify some-parts of the total design to connect it to reality.
The reason is that there can often be lots of decisions that need a large amount of context about the code base in order to reasonably determine what the right design and implementation is and this doesn't happen (in my experience) with the little Interview dialogue that pops up when it isn't sure. Or, even worse, Opus will just make decisions for you that are incoherent with the goals of the codebase.
So, I think, "plan mode" as an explicit mode is dead, but the act of planning is still a good thing to do.