In this message I'm trying to make the argument I think they would make (I personally use monospaced fonts).
It's not that Acme can't use monospaced fonts, it's that Rob/Russ/others don't want to use them. Proportional fonts are better fonts, so why not use them instead? One possible reason is that existing code formatting conventions assume that text is lined up in columns, but we have a tab key that magically lines things up: it's the whole job of the tab key. So, why not forget about space-based alignment, use the tab key for the job it was built to do, and get the advantage of using pretty fonts?
I used Emacs from 1993-2004, and switched to Acme for the 11 years to present. I don't miss trying to memorize all the key combinations from Emacs. I like that Acme presents a clean, simple, and direct Unicode interface to what I work with: mostly editing shell scripts, and running shell commands, as a build engineer. It takes a while to get used to mouse-button chording, but I don't even think about it now. I constantly use guide files, in many directories, to store and modify commonly used commands to highlight and run, so I make many fewer typos now, and don't forget which commands to run or how I run them. I can also switch contexts a lot faster, both because commands are laid out in the directories where I use them, and because the Dump and Load commands store and retrieve sets of files in the tiled editor subwindows. When I had to work on Windows I enjoyed having a pared-down unixy userland that I could write scripts in, to use also in my Linux Inferno instance (mostly communicated from one instance to the other through a github repo for backup and version control). The biggest drawback to me with Inferno is that so few other people run it, that I have to compile it myself on any new platform on which I run it (there are not really rpms/debs/etc available to just install it). But your experience with Plan 9 Acme might be better, I just prefer also working with the Inferno OS improvements, such as bind, /env, sh, etc.