I must be missing something. What does Secure Boot solve, that I care for, as an end user?
Like the now defunct hardware passwords on earlier Macs, the iMac Pro's Secure Boot does not prevent booting from another drive, it just adds an extra password step. It also takes at least some password technology out of software and transfers it to the arguably less vulnerable hardware hardware control.
Whether you would find it useful or not depends on you and how you use your Mac. Providing you keep no sensitive information on your computer such as identifications, passwords, financial data, sensitive project information, proprietary data,
etc on your computer probably not much. My take is: given the increasingly hostile environment we call
the internet and the target market for the new iMac Pro is the
PRO user who, by definition, will often have such sensitive information on their machines, it sounds pretty attractive — but that is just my opinion.
Personally I think it would be even more useful on MacBooks and MacBook Pros because of their far greater vulnerability to physical theft. My question is how long before Secure Boot makes its way to the downmarket iMacs and MacBooks. It is only a matter of time before facial recognition will have spread from the iPhone X to every MacOS and iOS device with a built in display and Secure Boot seems a logical expansion of security.