Sounds like forced indexing on members.
reCAPTCHA used for security and reCAPTCHA used for digitizing books are separate programs. When reCAPTCHA is used for security the application generates the distorted image much the same as CAPTCHA does. This is necessary to ensure there is an correct answer to compare a user's input against. reCAPTCHA used for digitizing does not have a 'correct' answer. Instead several humans are needed to provide answers. When enough answers that are the same are provided then a reasonably correct answer is derived from all the answers provided. This is similar to indexing in a way. Without a correct answer it is impossible to use reCAPTCHA for digitizing in conjunction with security. Users can input any answer because there is no 'correct' answer.
The primary development of reCAPTCHA was to help digitize books. From the description of
reCAPTCHA several humans need to decipher and digitize the text a computer cannot through OCR (optical character recognition). [As a side note: reCAPTCHA was originally developed to help digitize printed books. OCR for printed characters is less complex than OCR for handwriting.] A large number of answers are needed in order to reach a high confidence that the number of same answers has produced a correct response. No single answer is considered correct in of itself. If merely providing an answer is needed, whether correct or not, there is no validity to using this for log-in security. It is just an added level to the sign-in process that slows it down and complicates it.
I probably use the LDS Account log-in a dozen or more times on some days, and probably more on other days. A good number of times I am in a hurry to get to a site, get information or input information, and get off. Adding an additional step that can take several minutes, if not longer, while I attempt to transcribe some data that I may or may not be able to do may just add an unnecessary layer and frustration.
The usefulness of using reCAPTCHA to force indexing on members and still ensure valid or accurate indexing is questionable. Once a user determines there is no 'correct' answer and they are in a hurry or just do not feel like doing indexing at the moment, they will just input any answer to get signed in. That answer could be anything whether related to the needed answer or not. This adds useless data to the aggregation of data to be correlated. Eventually the number of answers needed to ensure a high number of correct answers would have to be increased. Currently indexing uses a considerably smaller number of transcriptions to determine a 'correct' answer than reCAPTCHA does for digitizing books.