If you wish to track data that is connected to my identity, i.e. provide continuity in your service, store some stuff that I made, settings, preferences, you don't need my email address.
If you really wish to communicate: Talk to me on twitter, it's nearly public! With oauth you receive a token as a representation of my identity, not my email address. If you really need an email address - then why do you need oauth?
If I go to a store to just browse through the shelves I don't have to give the clerk my telefone number. Why would he need it anyway? To annoy me every once in a while and bully me into visiting his store, which I voluntarily entered in the first place?
Here is a positive example: - Go to iron.io - Log in with your twitter account - Nothing happens: You can use the app and they show you around pretty nicely.
Yet Iron provides a service upgrade, which requires credit information. They ask for it, specificly when they need it: When you want more from their service. In that case you have to provide them with the info.
This is how websites could handle this issue. Log in with oauth. If you REALLY personally need to contact me, then you can ask me later. Not upon login or to distinguish me from other people or provide continuity.
Emails are communication handles. They shouldn't be a poor man's surrogate database primary key.
I remain very curious: What does _your_ service need my email address for?
Regardless of how legitimate you think my usage of email addresses is the inability to even ask for it _is_ a downside. For those use cases that do require it immediately you now have a system where the user clicks the 'Login without filling in a form' button and gets thrown to a form anyway, which is exactly what we were trying to avoid.
But do you have to do this whenever a completely new user tries to access the website? Is setting up an alternative access method not a very different concern?
