Single Opt-in vs. Double Opt-in

Single Opt-in vs. Double Opt-in has been the debate for as long as I can remember when it comes to list building and email marketing. The email service provider always wants the double opt-in for better list quality, few complaints & spam traps, etc. The email marketer feels they are losing out on valid subscribers if they use double opt-in since a certain percentage won't ever click the confirmation button. So the battle rages on...