I can work with any symbols as long as they are easily typeable. ++ is 3 easy
key press. `mappend` is 9. In both cases I don't need to look on keyboard as I
know exactly where they are. However there is no way I can remember where U+00A4
is - probably it is not on all keyboards (reversing " and @ gives me a lot of
problems in UK keyboards. FR keyboards was nightmare just because a few keys
where at different place). In fact I'd need to open keyboard mapping and search
for symbol. A lot of trouble just for single symbol.

To summarise - It doesn't matter is symbol is or isn't in ASCII or ISO 8859-1 as
long as it can be produced easily.