there are a lot of drawbacks to having mites:

1 - it's simply unpleasant to handle a snake with mites - they look terrible and they have BUGS crawling all over them - ew! then when you put them down you have these little critters crawling on your hands - heebie jeebies for dayyyys.

2 - if a mite problem goes untreated, you'll notice the snake becomes dehydrated and lethargic. I think mites like blood, so your snake would be prone to anemia with a large enough population of mites, and they start looking skinny and not gaining weight even before the mite problem is evident sometimes. It's not good.

3 - as has been mentioned, they carry godknowswhatelse that could be really seriously unhealthy for your snake. IBD is thought to be transmitted via mites, for example.

4 - they're majorly contagious. If you have even one mite in your entire collection, it's time to treat the whole collection. They spread like wildfire, and the only reasonable thing to do is treat everyone right away, for the reasons stated above.