Sure, you could indeed use markup to make the burden of uniq-ing lower - that mitigates the impact of the design decision, but doesn't address the underlying issue.
Fundamentally, hygienic macro systems assume that capture is unintentional by default (although you can ask for it), rather than assuming that capture is the default and you have to ask to avoid it.
When writing macros, which is the more common default? Since arc appears to be using macros extensively in it's implementation, perhaps it was found that capture was actually more common but that implies a language implementation issue has been 'promoted' to a language design issue. Maybe I shouldn't see those as seperate - this is LISP after all :-)
Hygiene is one of those things that doesn't bite you when the code-base (or more importantly, the idea-space) is small. But as the system gets bigger (and I'm assuming we all want arc to be good for big problems), you can't keep it all in your head anymore and spending many hours debugging a complex application to find a simple macro capture oversight bit you is not fun.