He insists on wrapping electrical tape around switches when installing them and around wire nuts. Is this good or bad?

Don't know about switches, you have different ones over there to the type that we have, but with wire nuts it wouldn't be a bad idea. I am not a fan of those things at all, they're ok for temporary lashups, but as part of a permanent installation..? Not for me, thanks 

The utility here used to close cobra heads with faulty latches and ballast covers missing screws on ballast-in-arm fixtures with electrical tape. Most held quite well. There is even a Cooper Vanguard that was missing its lens and ring assembly. They put in a new lens and a good wrapping of electrical tape, and it held quite well even during hurricane Dorian.

No tape on any of it. If you think you need to tape it then that just means you're not confident in your installation.

It’s a prophylactic to shorting something out, especially when pushing the unit back into the JBox. It’s not a bad idea in moderation. Let’s consider the fact that in the US the live and neutral are exposed on the sides of a switch or outlet as soon as the cover plate is removed. Here’s a photo to illustrate my point, those exposed screws on the side are LIVE. Oh and unfortunately here wire nuts are code!

It’s a prophylactic to shorting something out, especially when pushing the unit back into the JBox. It’s not a bad idea in moderation. Let’s consider the fact that in the US the live and neutral are exposed on the sides of a switch or outlet as soon as the cover plate is removed. Here’s a photo to illustrate my point, those exposed screws on the side are LIVE. Oh and unfortunately here wire nuts are code!

A code I've not deciphered, fortunately  

Live exposed screw terminals frighten the **** out of me, although the lower voltage will help, you still get a belt if you touch it  

Pretty common in the industry to wrap/cover the exposed wire terminals/screws on switches. Especially when having to work on live circuits.  I do it myself often to prevent arcing when trying to get a switch put in next to another switch or in an old metal switch box. 

To cover live things yes, but use something sound and not just a tape, which slips away after few weeks when not applied 100% right. The tape is designed as a "necessary evil" for "last line" fixes, but for first line installation you have use something more sound.
The fact you feel the need to use it on a new installation means you are doing something intentionally not right in the first place. And that is the problem (not the tape itself).

And don't tell anything like "what is so difficult to use a tape". It is way too easy to overlook spots where it slips away after some time. To use it really correctly, for each individual shape you need to have exact wrapping pattern, already proven it wont unwrap and stick to it (what is the case for some industrial mass production, where the tape is sometimes used). But for an arbitrary work you have no chance to know, if the pattern you have just used will remain stable or not without "giving it the chance" to happen over time...

To me presence of electrical tape is the signature of "we dont need a code" installers and "fly by night" electricians

I work where we have an actual problem with electrical tape.

As a traffic signal electrician, I see way too much electrical tape being incorrectly used. In Seattle, it rains a lot. Other techs will wrap electrical connections with electrical tape. What happens is that the electrical tape traps and holds water at the connector, which prevents the connector from drying out when it's not raining. This will QUICKLY destroy a connection. Even MS series "mil-spec" weatherized connectors will fail if not allowed to "breathe".

As for in-wall electrical devices, I see no real harm in taping the screws. But, for the love of God and baby Jesus, just use good quality, non-Chinese electrical tape. 3M 88 or 35 is good stuff and won't slide-off the electrical device over time. Me, personally, I don't tape those screws, but I do drive them all the way in if I am using the "back-stab" connections. (yeah I know, those are another topic altogether)

Regarding the OP's dad being a "tape freak" I like to correct people any chance I get. Sometimes it takes 5 years to get people to see the accepted way of things. I had a co-worker, who despite getting paid $120K a year to be an electrician, still DIDN'T KNOW HOW TO USE CRIMPERS!. He would use the toothed "non-insulated" jaw of a crimper to crimp insulated terminals. He LIKED to see the crushed terminal body to, in his mind, confirm the completeness of the crimp. What he didn't know, was that the "non-insulated" tooth was splitting the terminal ferrule at the seam and his wires were actually very easy to pull out. Literally took 5 years, but he finally accepted that was the wrong way to do it. BTW, I see this incorrect method used very often.