I can drive my car for years without wearing my seat belt and be completely successful doing so. Does that mean I shouldn't wear my seat belt?
There is RISK to not quarantining. How great that risk is, depends on how trustworthy the source of the animals is. Just like there is risk in not wearing a seat belt. How great that risk is depends a lot on who is driving, and the circumstances they are driving under.
You can choose to never QT your animals...and if you're lucky, it will never be an issue for you. If you're unlucky, you may have to deal with a serious mite infestation or something. If your luck has completely run out...you could lose your entire collection.
It's all a matter of choices...how much risk do you choose to accept? How much risk do you choose to take steps to mitigate? Even with QT, there are never an guarantees. There's always risk of some kind of contamination when introducing new animals into your collection. But with a solid QT, you reduce that risk considerably.