namespaces in combination with how syntax-quote (`) autoqualifies symbols and let disallowing qualified identifiers makes it noticeably more difficult to introduce a name clash.

for example the following macro results in an error when you try to run the expanded code: (defmacro ex [x] `(let [y 1] (~x y)))

because it expands to (let [namespace/y 1] ...) which is disallowed. When you need to, there is a way to force an unqualified symbol by explicitly quoting and unquoting a symbol, and ` provides autogensyms, so it is very convenient to use.

Namespace-qualifying also means that post-expansion changes to your local namespace won't change the meaning of symbols that referred to things in an external namespace when the macro was expanded.