The whole "Apple refused to repair my iMac" has blown up.  Anyway, here's my stance.

Apple not having the proper support for repairing a ten thousand dollar all in one seems insane to me.  Those who are defending Apple are stating that their terms of service state that they don't have to.  Although that point is irrelevant since Apple did seem to try to repair it.  Also, that's more a legal disclaimer to cover their asses.  Most companies have something similar, and most companies try to repair, since it's usually cheaper, and tends to make the customer happy, thus helping ensure repeat business (and to be fair, that does seem like what Apple was trying to do).  Stating that you aren't going to repair it, and you legally don't have to is a surefire way to loose repeat business with said customer.

Although those who are using that excuse are pissing me off, it makes Apple's service technicians out to be complete dicks, which isn't the case.  They are trying to twist the circumstances in Apple's favor, yet ironically, making Apple seem even worse in the process.  Being incompetent is better then being an asshole, and is usually easier to fix as well.

My biggest criticism to Linus was buying a non self serviceable product in the first place.  At least one that was that expensive.

I break my desktop screen?  I can replace it without having to replace my whole computer (that is, if the company in question refuses to fix it).  Same goes for any other component in my machine, motherboard, RAM, CPU, GPU, hard drives, etc.

It's idiotic to spend that much money on ANYTHING that doesn't have standard replaceable parts (ATX/ITX style motherboards, PCIe slots, etc), period.

Be it Apple, ASUS, Intel, MSI, AMD, anyone.

So, with that said, why should I ever invest in a iMac Pro over an equivalent prebuilt?

The only reason that I can think of is for the OS, and there are very few OS X only applications that make the OS necessary.