He’s not just a proponent of message passing, he’s a proponent of late binding [1]. The idea there is to have a running image of the environment and interact with it in real time, updating code while it’s running (no recompiling or anything like that). The idea is a high level of interactivity with very tight feedback loops.

A whole operating system like this would allow you to hack on the user interface and change things on the fly. All of your software would also work like this, promoting open extensibility by the user. You ought to be able to click on any window or other user interface element and be able to view and modify the running code live, without even restarting the computation it’s running (never mind the whole program or even rebooting the computer).

This sort of ability to do live hacking on the internals of what you’re working with is how computers used to work in those early days. It’s also how machinery has always worked in the past. A mechanic could lock in the timing of an engine by rotating the distributor cap and listening to how smoothly it fires.

[1] https://en.wikipedia.org/wiki/Late_binding