It comes down completely to what you can do with those users. If you have a good marketing strategy which will focus on marketing to those people via email and on the social networks where they follow you, then by all means, definitely start collecting the emails (and eyeballs) as soon as you can.
But with that said I've seen too many times, companies collecting hundreds and thousands of emails, not just from some vague email form, but from people who explicitly specified that they are interested in the company's product, and they just let those emails sit there.
...and by the time they get around to start communicating with the people that signed up, no-one even remembers who the company was and is wondering how they even got their hands on their email.
To summarise, if you collect eyeballs, use them quickly and efficiently, they don't have that long of a shelf life before they go bad.