I read somewhere recently (and I maybe wrong) that in Germany double opt-in is mandated under law now as it stops nefarious activities by others. This also limits risks to your servers from hackers attempting to register millions of users all at once :)
Requesting and email address and verification also confirms that the email provided is legitimate so if you want to contact users in the future for other reasons (and marketing consent is agreed to by the user) this could be a valuable commodity for the business.
I agree with funkylaundry that postponing sign up as late as possible is often a good decision as users will become familiar and engaged with the product and see eventual sign up as less of a barrier; but how different is this from a trial?
One solution (that i think Myfitnesspal use) is to let the user sign up very simplysimple (Username & Password) and have full access to the site but regularly remind the user to provide and verify an email (potentially with a 'do not ask me again' option). This presents a fairly non intrusive solution.