Captcha adds unnecessary friction to the checkout process on any website. In fact, it's been shown to lower the conversion rate in published studies. That is why you never find a Captcha associated with any commercial shopping cart. Since registration spam is never a problem on any checkout page that requires the exchange of money, it would be nice to allow MM users to install Captcha only on free membership signup (checkout) pages. Thanks for considering this suggestion for future updates.
Thank you for this suggestion! This feature request or some aspect of the functionality suggested is currently being reviewed for possible inclusion in a future update. If there is additional information, this post will be updated to reflect it.