In general, lack of encryption enables passive surveillance. I think a better solution than opt-in encryption could be on-by-default, with a multi-party recovery scheme.
> If you're using OS X Yosemite or later, you can choose to use your iCloud account to unlock your disk and reset your password
> If you're using OS X Mavericks, you can choose to store a FileVault recovery key with Apple by providing the questions and answers to three security questions. Choose answers that you're sure to remember
> If you don't want to use iCloud FileVault recovery, you can create a local recovery key. Keep the letters and numbers of the key somewhere safe—other than on your encrypted startup disk
Which is exactly what it's supposed to feel like, and that's fine if you were aiming for it, but when that isn't what you signed up for, it's pretty much the worst thing ever.
It can also feel like a truly nefarious form of vendor lock-in, if you ever try to migrate to another system, and just happen to notice that now, your disk is totally unmountable for some unexplainable reason. This kind of thing can happen when crossing over to an older version of an alternative OS that doesn't know how to negotiate authentication for proprietary encryption formats, or newer formats that weren't supported at the time of the older OS's install.
So, for consumer hardware that might be received as a birthday gift, I'd say encrypted by default is a terrible idea. For those not carefully and explicitly warned, storing their stuff on an encrypted disk gone wrong, can be like unintentionally throwing it in the trash, setting it on fire and then flushing it down the toilet, except the toilet is a black hole of no return.
We're talking about an Apple iMac Pro here... this isn't a computer that is designed to allow you to move components to a different system. Yes, it is a fully locked-in system, purposefully designed to be as secure as possible. I'm not going to give Apple much grief about that, because it's not like they say you can migrate hardware from one system to the next.
It would be more interesting to know how the T2 chip interacts with external storage and/or alternative (Windows/Bootcamp) operating systems. In both of these cases, one would expect more low-level data portability, which is a more interesting use-case than trying to pull out the NVMe SSDs from a glued together iMac.