What do you gain from having an e-mail:
- usually nothing, unless you send your users spam.
What do you lose from having an e-mail:
- around 10% of your potential customers. (the number probably varies wildly from site to site)
- People who lose their password can't get into their account, unless they choose to provide an optional e-mail adress.
To me that is a pretty clear choice.
They should have a "Don't mail me bro" checkbox. I don't always agree with their definition of "spam".
There might of course also be other valid reasons for wanting an e-mail adresse.
I think that the metrics support this view as well.
Also if you are running a service with termed licensing, an email communication is how you drive the license renewal process. This sort of communication does not qualify as spam, because it is more of a service reminder rather than an unsolicited commercial offering.
I was referring more to the sign-up stage though, where you tend to lose a lot of potential customers with obligatory confirmation e-mails.
But if you can get the conversation going after the critical sign-up step, you should by all means do so.
All your users have it-- have you thought about when your users might like to be notified about new activity relating to them? (opt-in, of course)
You just shouldn't request an obligatory email adress, or even worse a confirmation by email, unless you really have to. It tends to scare a lot of users away.