I bow to your greater experience, however the problems with a DIY kit are (a) knowing what you are doing (b) the pressure gauges being unreliable (c) the more commonly available regassing kits also contain a sealant to seal any potential leaks (even if you don't have one!) which then bungs up the compressor (d) the kits are more expensive to buy than a full recharge. (e) You can risk an expensive bill to fix the system if it all goes wrong.
You just have to accept the facts that
1. they are available, and you get what you pay for, buy a cheap filling hose and you get an unreliable pressure gauge.
2. most people with common sense and an IQ higher than their shoe size can use them successfully, just follow instructions on the bottle.
3. they are not expensive if they last 5 years+ for two cars.
4. that systems only rarely need a full excavation / recharge
5. most systems lose a small and fairly consistent amount of gas each year
6. there is no mystery about aircon (much as people who make money out of it would like to create one) it is just transfer of energy (heat pump) by gas getting compressed and expanding, just a bit of physics that makes our lives more comfortable.
7. the DIY kits are not meant for a full system excavation and recharge - just the occasional top-up.
8. I have never destroyed an aircon system and needed to pay for expensive (or any) repairs
9. I have never to my knowledge had a 'bunged up' compressor.
System performance is degraded when too much gas in the system as well as (more commonly) too little gas, the gas needs correct free volume inside system to expand properly after it has been compressed, that is why a full recharge needs a known amount of gas by weight to be put back into system, and a vacuum is used to make sure any substance in the system (oil etc) is evaporated and taken out before fresh is put back in. Most reputable leak sealers react with oxygen or moisture in the atmosphere on the outside of the system to seal leaks and do not cure inside the system.