I know that my most recent case does have some interior LEDs of the type described, to show off all the extra cooling fans.
But I would love to get a computer with extra LEDs of another sort. Imagine a microprocessor chip with a documented diagnostic mode, so that you could flip a switch, slowing the computer down to a stately 100 MHz or something like that, where it used the extra bus cycles at 1 GHz or whatever to export the status of its various internal registers.
Imagine it being housed in a case and motherboard that let you see an old-fashioned gingerbread front panel while the computer was running when in that mode... not just lights for the bus, like the old Altair computer, but lights showing how the pipeline, the cache, the floating-point unit, and so on, were individually functioning.
And, of course, switches to slow it down further or single-step it.