Oliver said:
I think one thing we may need to consider is what this binds us into. So, there's a very good reason as to why a lot of the old guard have disabled email or not provided accounts - the emailing system built into MediaWiki sucks. It can be, will be and has been used for wide-scale abuse that it's pretty difficult to shut down.
This is a good point. In my work with subject recruitment policy in Wikipedia, I worried that using SpecialEmail to contact users circumvents the primary means for Wikipedians to notice when other people are being spammed. For example, if I receive an email from a potential spammer, it's currently impossible for me to tell how many other users were contacted, so its hard to know if I should just make a post in the Village Pump to see if there are others or quickly find an admin on IRC to shut this user down. This is my primary reason for recommending that subject recruitment requests happen on wiki -- so that Wikipedians can most easily track its use.
If we mandate email we're also dramatically sizing up the pool of people that bad-faith trolls can take a shot at; we may find the community demanding a rebuild of Special:EmailUser when the vandals of the world notice what we've done.
I just ran a few queries to check how much we'd be sizing up our pool of emailable editors. In the last year, 380,281 users registered an account and made at least on edit. Of these users, 233,733 (61.5%) provided an email address and 128,572 (55.0%) confirmed it. Assuming that the same number of people register accounts once we require an email address and the same proportion of people that provide an email address will confirm it, requiring an email address could increase the pool of email spammable editors for new cohorts by about 80k users (a 63% increase) from 128k to ~200k/year.
-Aaron