Diego certainly makes an excellent point that we are already authenticated via email address and password authentication, so why would authenticated users be required to enter captchas? The captchas I see (on my 24” widescreen monitor) are hard for me to read. I wear glasses for astigmatism and nearsightedness. For really low-vision users the task must be difficult indeed. If you happen to be both low-vision and deaf, I think you are really out of luck with these captchas. Plus making your users do extra work in order to express themselves…I don’t know.
I am sympathetic to the spam issue, but I do not think Captchas are a good solution. They look interesting at first but then they squeeze out small but important groups of people from posting. And sooner or later someone will figure out how to programmatically determine the content of a captcha and supply it for processing.