This is a good idea and would be easy enough to do so I will add it to my near term to do list. In general though I've been on hold for doing any enhancements to the blog comments because I plan to build a better comment system that can be used across features and then swap it out to the new system in the blog when the new comment system is ready.
But this will be easy so I will go ahead and do it since it may be a few months before the new comment system is ready. In this case if the user is authenticated we won't need the captcha so I will hide it when this setting is enabled.
Best,
Joe