That doesn't mean that a gem should come first. DDH became famous among other things for his "opinated software" policy. By the way, I've been enjoyed a lot his recent book "Rework", although I've not finished it yet. Highly recommended. As well as Jos� Valim's excellent book "Crafting Rails Application".
Back to the subject, after the free merchandise, what I'm saying is that this is a very common requirement and most developers don't (shouldn't?) care that much about e-mail format. At least for 90% of the applications. So, I don't see any reasons why not to include some "validates_email_of" helper. It doesn't need to solve all cases. It can have some options for specifying some regular expression or some lambda for verifying the e-mail format in case someone is really worried about it. It can even be customized in a global configuration per project/engine/mountable application. There could be some pre-defined options specified by symbol like :rfc_compliant, :performant_verification, etc.
I just don't buy the idea to make this more difficult than it should be. It is much simpler to evolute and understand better the validations after they are provided on rails core distribution. Also, in the beginning, there were lots of philosophies about Rails, like the 80/20 principle. No body talks about this anymore... In every web application with a user sign up feature, there will be an e-mail validation. This is probably over 90% of the web applications built with Rails. Even so, this will require all of these developers to replicate the logic to validate e-mail. Most of them will just extract this one from Rails documentation:
validates_format_of :email, :with => /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i
That is because, this is usually the first thing written in a new application that requires a user to log in. In this stage, no one is willing to invest any time on the e-mail validation rule. Usually this will only happen if some customer complaint about not being able to register himself because the system is rejecting his e-mail.
For my applications, I usually don't care very much about the format because of the way I handle the registration part. The user doesn't input any information in the register form, except his e-mail address. Then, he is informed that there were sent instructions for continuing the register process to his e-mail and he'll continue to register following a link on the e-mail. That way I can know for sure that the e-mail is valid.
So, I think that even if not perfect, Rails should not wait for any gem before supporting e-mail and URL validations built-in, even though I have a feeling that it won't happen based on what I've being observed from the core-team point of view.
Best regards,
Rodrigo.