We can demonstrate the non-strictness property in the usual way:
[(_ exp) 3]))
This non-strictness is critical to Scheme in a couple of ways--and the reason is always that we need to see the original syntactic structure of an expression. The first case is for uses of quote: if (foo exp) expands to (quote exp), then (foo (or a b)) should expand to (quote (or a b)), not (quote (let ((tmp a)) (if tmp tmp b))). The second is for macro-defining macros. If a macro expands into a definition of a new macro that needs to pattern match on one of the original macro's arguments, then new macro should see that argument as it originally appeared, not some expansion thereof.
I believe that the latter would not be an issue in so-called generative macro systems, where macros are not allow to deconstruct their argument expressions.