It's not a completely non special character: for instance in bash it's special inside braces in the syntax where "/{,usr/}bin" expands to "/bin /usr/bin". But the need to start that syntax with the open brace will remind you about the need to escape a literal comma there if you ever want one.
Oh, that might be true, I do remember encountering some escaping issues when creating a more complex POSIX (or bash) script that involved lists and iterating through stuff.
I see Bash only uses commas in Brace expansions:
file{1,2,3}.txt
# file1.txt file2.txt file3.txt
I guess it would only be a problem if you want to expand