We could just say, macro! token get's desugarred to macro! { token }. That way any macro can take a single token and not need unnecessary delimiters. (This may even be nice for custom literals, say bigint!120381204803284123, or s!"hello world" for String literals)
This way if you need to call a macro with multiple tokens, yes you will need delimiters, but only if you need multiple tokens
edit: I'm going to say a single token is something matched by the tt macro specifier, but excluding grouping tokens for back-compat.