Looks like they're ultimately throwing their long-term business under the bus for a short burst of extra cash.
If you use MailChimp and care about deliverability, I would recommend you begin looking at alternatives (if you haven't already).
Yes, most others have already switched to single opt-in.
> It's not all of the sudden worse than alternatives, right?
No, they aren't all of a sudden worse than the alternatives. They were already worse and they're certainly not going to improve as a result of this change.
I manage several e-mail servers and will be watching very closely -- even more so than usual -- the amount of spam coming from MailChimp. I usually start out by blacklisting individual IP addresses but if this becomes a big problem, I will simply blacklist them entirely and be done with it.
I've had two "incidents" in the last week or so with MailChimp that they have shown little interest in "fixing" (unrelated to this change) so I'm already inclined to do exactly that.