From 'Let Over Lambda':

These solutions try to limit or eliminate the impact of unwanted variable capture but unfortunately do so at the expense of wanted, desirable variable capture. Almost all approaches taken to reducing the impact of variable capture serve only to reduce what you can do with defmacro. Hygienic macros are, in the best of situations, a beginner's safety guard-rail; in the worst of situations they form an electric fence, trapping their victims in a sanitised, capture-safe prison. Furthermore, recent research has shown that hygienic macro systems like those specified by various Scheme revisions can still be vulnerable to many interesting capture problems