Those statements are just wrong. Blanket statements like that usually are. THE ONLY thing going thru the switch is the headlight power line, so unless you wire into the headlight power any other wires in the bucket are not going to affect the switch operation. Nothing else goes out other than the headlight when you hit the switch so therefore nothing else is going thru the switch. Just look at the electrical diagram its only the headlight power. In addition, the headlight power line is fused at 10A, I don't know of anyone that adds directly to the headlight line and also ups the fuse to compensate for the addition. That switch is on a 10A fused line so by safety design standards the switch would be required to handle at least 10A.
The issue is cycles. Everything electro/mechanical has a life cycle. Everyone knows when you throw a switch you get a little arc, a light switch at home, an electric drill, just disconnect the battery in the dark and touch the terminal with the cable and even the odo holding current will give a little snap. You do this enough times at several amps and the surface of the connection erodes away, continuing the surface erosion you end up with a smaller conduction path, at some point the surface area can't handle the current and now you have excess heat caused by the resistance, current trying to conduct thru too small of an available path.
Regarding the relays, not everything requires a relay which is why many electrical accessories don't have them. It all comes down to what end function you desire and then design it soundly to meet that function. Other than the engine electrical requirements everything else is considered an electrical accessory on a vehicle. For example our horn works absolutely perfectly without a relay, but a lot of vehicles have a horn relay, put a stebel on an x and it is a good idea to do it through the relay. Why doesn't our horn have a relay? because the horn switch is adequate to handle the current of our little squeaker so a relay is not required. Stebel startup current is pretty high so you don't want to run it thru the stock switch.
I could give a lot of examples where a relay is desirable and where it isn't. For simplicity a relay is a switch and in automotive uses its primary purpose is to act as a switch, either as the main switch or to protect (bypass) a smaller switch. If you don't want/need it switched or the existing switch is adequate in size then a relay is not needed.
A CB radio for example. I wouldn't want that switched through a relay because I want to be able to still use it if I'm broke down on the side of the road without turning the key on. And it doesn't need another switch since it already has its own correct rated switch built in. Similar with my accessory power port, I don't want it switched because I plug the tender in there, I've used it a few times with my air compressor for flats, and I run my computer through an inverter off it when I'm in the tent. It is direct fused to battery.
