Wait, you don't really mean that, do you?
Diagnostic LEDs -- for example? I can't see through aluminum, and I don't think you can either.
I've worked on plenty of powered-up computers, and seen plenty of well-trained techs do the same (for example, techs from the late great Digital Equipment Corporation). It was a DEC engineer who taught me to use a cable tie to test whether a hidden fan was spinning or not.
Because everybody's guessing, my guess is that it has to do with properly isolating/insulating AC. I've been into tons of boxes (typically not computers) with interlocks that break the AC connection when the box is opened, and of course the Mac Pro doesn't have anything like that.
This (positive interlock) was especially common on old tube TVs (yeah, I'm not young . . . I disassembled my first TV back in the 1950s), even though the charge in their capacitors could easily kill you.