He is about 15 to 20 years out of date on this.
Firstly it is complete unnecessary, modern electronics are designed power down when not in use, or to be switched off.
Secondly it is environmentally unfriendly and potentially expensive on the electricity bill.
Thirdly it will collect more dust, increasing the likelihood of failure.
A little knowledge is a dangerous thing.
There's the classic story, of the Mac Plus with an internal drive (no, not an SE20) that ran continuously serving the Apple dealer's internal messaging, for over a decade. They had a power out one night, and the thing wouldn't start. Turned out the hard disk drive wouldn't rotate - the spindle seemed welded in. By leaving it on, they got years more out of the drive!
However, things do change. And the wear in drives is much more likely to be dependent upon how much the heads have been accessing the platters. And the amount of time the drives are active and hence rotating. While they are idle, they are not moving, hence not wearing.
Of course, there are also motherboard issues. Once a common problem was "dry solder". I don't know if that still happens???
The argument for leaving things on is both current surges, and also having more stable temperatures.
For long life, the best thing would be to have the boot drive solid state ie SSD, and to put a UPS in between the power and the computer. And them probably, it just might last longer turned on that off.
I doubt its worth it though. And with an SSD, a machine boots quickly too ... which would likely encourage more frequent shut downs and hence start ups. But curiously, that is when a lot of the stress comes - the thermal expansions happening from going from perhaps quite cold, to average operating temperatures.
I think too, that electronic devices fail in the short term, or last. So for a new Mac Pro, IMO its best to leave it on for the first month anyway. Just to encourage the very unlikely but still possible fault to reveal itself earlier, rather than one month after the warranty has expired.
Now ... how can we make our monitors retain their new looks for a decade???