Optimize for a few hundred spam obsessed power users, or, prevent a major cause of the #2 most common CS complaint at many businesses. This does not take much pondering.
P.S. Trivially A/B testator at high volumes if your CS infrastructure is capturing sufficient data.
Which part of this is the #2 complaint - users making a mistake entering in their email address? Isn't this why you ask them to confirm it by sending them an activation email?
The activation email isn't a panacea for users fumble fingering (or misremembering, or not knowing) their email address. Users who don't receive it will either a) ignore it if you let them use the application anyway or b) frequently bounce hard if they don't get it, because they assume naturally that your Googles are broken.
The single most compelling reason to send people activation emails is -- I kid you not -- to remind them that they signed up for your website and how to get back to your website. A secondary consideration is not proving that they got their inbox right but proving that they didn't get someone else's inbox wrong.
P.S. Trivially A/B testator at high volumes if your CS infrastructure is capturing sufficient data.