One of the most visible uses of Show is that it's how values are shown in GHCi. As mentioned earlier in this thread, if you're teaching in a non-ASCII language then the user experience is pretty poor.

On the other hand, I see Show (like .ToString() in C# etc.) as a debugging tool: not for seriously robust serialisation but useful if you need to dump a value into a log message or email or similar. And in that situation it's very useful if it sticks to ASCII: non-ASCII content just isn't resilient enough to being passed around the network, truncated and generally mutilated on the way through.

These are definitely two different concerns and they pull in opposite directions in this discussion. It's a matter of opinion which you think is more important. Me, I think the latter, but then I do a lot of logging and speak a language that fits into  ASCII. YMMV!