It doesn't seem a good idea to use characters that have special syntactic meaning in the names of things. You can't name a function foo(bar either, but that doesn't seem odd because Lisp hackers are used to ( being different from other characters.
It seems too that special end-of-symbol handling in the core, just because some users might possibly want to use intrasymbol-notation characters at the end of function names, does not go well with Arc's design philosophy:
... avoid having a language designed according to arbitrary rules rather than the actual demands of the task ...
This is potentially an issue if you open up intrasymbol syntax though. It'd be pathologically dumb, but what happens if I set up - to be an intrasymbol macro?
Solution: perhaps user defined intrasymbol macros should be surrounded by colons (:-:) by convention? Do intrasymbol macros support multiple characters?