I am not particularly asking about whether or not Captcha within itself is good UX, but a more generalized question as to when human validation truly makes a difference in terms of a website's integrity, and what "kind" of website those may be?
For example, websites (e.g. social networks) where user accounts are forward facing, and friends/groups are involved and most importantly the integrity of the website is dependent upon user generated content.
An ecommerce site where user accounts are not accessible to the general public and the end goal of the site is to convert via a credit card being used thereby ensuring that the user is a real person (in theory...).
Also are there other instances besides social networks / ecommerce, e.g. Software as a Service, where captcha is essential or not essential.