After all biocides cost next to nothing and provide peace of mind, so why not?
Because they don't cost next to nothing. They cost at least a couple of pounds for a couple of pennies of chemical and it's just something else to scare newbies with. And these chemicals are poisons, so they're not the sort of thing you want lying around the house if you have pets or children.
And it also allows the manufacturers to cover up the whole distilled water scam. Most of isn't distilled water at all. It's deionized at best but most people don't know the difference.
Even then, there is no source of nutrition, so what's going to grow? Nothing.
The only thing disturbing my peace of mind is the advertising blurb from the manufacturers.
Don't use a kill coil because it could react with our badly finished equipment.
Don't use anyone else's fluids because we can't guarantee what will happen. Of course they know exactly what will happen but because they buy from Chinese manufacturers that fake certificates of compliance they're no more manufactured to a specification than fly to the moon.
If your tubing goes cloudy, blame the tubing manufacturer because it's easier than testing and reformulating your product so it's compatible with all the popular brands of tubing.
The manufacturers, by and large, just want to sell us product. They have no interest in making it last because they want us to buy MORE product. What, you're still using our 2011 CPU block? You poor thing. Our 2012 block is flavour of the month now you know. Buy that and your PC will run colder than Antarctica. It doesn't of course, but we're all mugs to marketing. Even me.