Oddly enough, everything I've read and all my experience disagrees with this.
It's not a good idea to use AQ salt (and never use table salt!) as a preventative, as long-term it can have negative effects on your betta's internal organs (I think it's the kidneys). It also causes them to build up a tolerance to it, so when you use it medicinally, it is less effective. It should only be used as a medication, not as something you add regularly.
Epsom salt is not harmful when used over a longer period of time. My last girl Artemis suffered from internal parasites, and I kept her in a tank with 1 tsp per gallon of epsom salt for a 15 day run in order to get her to expel them. It was perfectly safe, and did no damage to the other fish in the tank. I suspect something else was wrong with your betta that suffered in the bath - perhaps shock acclimatisation, or some other cause.
It could have been shock, but from what I wouldn't know. The water was the exact temp of his tank water and everything, I tried it a couple times a day for a few days and it was always the same. He didn't have that problem before we put him it, he just started to sink to the bottom after about 20-30 seconds, then lay on his side and appeared to be gasping and very stressed.
I wonder which is right though, because I've also read the opposite of what you say. Just like everything about aquatic animal care, there seems to be a lot of conflicting information about proper care. Funny enough, it was people in this forum who first suggested I add a bit of aqs regularly, which is why I researched it.
Though your point on becoming immune to it does make sense. The only thing is that since I started putting aqs in my tank, my betta has become more active. But in the end, I find scientific journals to be much much better to reference than people online, so if you know of any, I would love to read up on it
When there is such conflicting information, and all of it seems to come from different "reliable" resources, it seems the only way to decide is to dive in and get to the actual studies behind it.