When letting Google or Yahoo store the email in the cloud one assumes that _they_ keep a backup - indeed that there are several copies of the messages stored - so this seems weird.
A lot of people have said that our brains are wired differently. I highly recommend Delusions of Gender by Cordelia Fine, it's looking into exactly those claims:
> I understand emacs's architectural reasoning for it -- and that is a choice.
I think the choice they make is to spend their very limited time on more useful features. I'm sure many of the core Emacs developers would be very happy if someone would fix this limitation. It would be a very invasive change, a fair amount of work.
So "opt-in by default" doesn't make any sense.
However, I do see this opposite usage increasingly often, which means "opt-in" shouldn't be used any more.
Using "on by default", "enabled by default", "off by default", "disabled by default" would be clear and make sense.