two points
1. you can run rio in a rio window so you can debug
rio without fiddling with your root rio.

True, but what happened to me was that I did not want to do any debugging. I wanted to work, i.e. use the computer in the conventional sense -- do sth. for my school. And you do not usually run rio in rio just for fun in that case. And then sth. bad happens. And you are ... Actually, this might just have happened when the bug we're talking about was found -- by chance, it just happened unexpectedly.
On the other hand, If you know in advance... you run rio in rio. But again, not my case.
What I am still convinced about is that always having a way to run a shell is invaluable.

2. if you're not running the fileserver on your cpu server,
it's always safe to reboot. in fact, the dozen or so cpu
servers that i manage are rebooted by pulling the plug.
this is one of my favorite features of plan 9's architecture.

Yes, I know.

i'm not sure why you can't have a dedicated fileserver

It's not a question of money... but thanks for your recommendations. :)
The main reason is I run plan9 only on one of my notebooks (otherwise I use linux machines since plan9 doesn't have a bit of the software I crucially need). And I want to be able to work on it, even though it is not at the moment connected to any network. I need to have my files with me. Is that enough?

So once more and to conclude, wouldn't it be nice to be able to press something like ctrl-alt-backspace to, by some means guarantee a way to a shell?

Ruda