Om, while talking about Ning, reiterated a commonly held idea I kind of disagree with:
you can't move your users with you, basically handing over your community to the company
If I use your site, I don't think having my email address actually means very much. Heck, for any non-transactional web based app, it probably will never be used except for things which land in the spam bucket. This is triply true for anything community-centric.
I am a member of lots of online communities, ranging from things like Honorable Players (one of the longer running gaming clans around, even if I don't have time to play anymore), to Apache (whom you probably know if you read this blog), to LtU (despite the fact I am 90% lurker there) to The Order of the Stick (not the fan community, just a daily visit and something I am attached to). I think only Apache has a working email addy for me, but most of the HP folks probably have one that will wend some path to my inbox.
More importantly, if any one of them were to move, say to Codehaus in the case of a subset of the Apache community, I would just follow the community, not the person who knows my email address or has the domain a community I love happens to be using right now.
For a business, being able to uniquely identify you far and away trumps having your email address. The ad networks nailed this one. Having an email can even be a liability -- people make throwaways and you wind up mucking up your internal metrics because folks are afraid you'll sell their email address to those dastardly spammers (not a useful business model anymore) or something worse. So, you can uniquely identify people -- all this takes, in practice, is a cookie. If someone wants something more resilient they can trade the cookie for a login/passcode combo so they can get a tighter grip on their cookie. If they want you to tell them when interesting things happen they can give you an email address. Asking for the email addy then lets you provide me an additional service, it is the opposite of "having my email address makes me your user."
An example of some folks who seem to very much get it: Meebo. You can go and start IM'ing through them without ever creating an account. It works fine (shockingly well for a web app doing what it does, actually). If you use it enough you tend to create an account just so you don't have to keep logging into multiple IM services through them. They have a ferociously loyal community and... they could pick up and move to wobblywombats.com, erase the user database, blog about it in a couple places, and aside from a couple clicks of confusion, not miss a beat.
I suspect the knee-jerk reaction to say "if you don't have their email you don't own them" comes from the Dark Days of Internet Advertising when it amounted to getting the company announcements or having a point of contact for the Sales Guy. Now... heck, you are better off forcing them back to your site to see ads, actually. Heh :-)
As a thought experiment, what would I lose if I stopped using personal email? To qualify -- I need to use it internally at work, I need to use it for open source stuff, and I probably need it to handle receipts for some internet purchases. Instead of having complex rules I go to straight white-listing. I can still sign up for any service which thinks they need an email to own me -- mailinator solves that nicely. I am still in touch with friends and family -- we use IM and this magical device called a "telephone" -- you probably have one built into your SMS/Camera device. I would stop getting those annoying chain letters. I would stop being contacted by recruiters who think I am perfect for this .Net contracting position requiring two years experience and a CS degree. Actually, scanning my last-week-archive the only thing I would have missed out on is the Adequate Guinea Pig. This deserves thought :-)
