Underwear or not underwear?

I often read people saying: "girls should not wear underwear with a chastity belt".
Some women consider their belts are underwear, I understand that.
Some "doms" say the same thing, I don't understand how they could say this at all.

Personally, I consider my armor as a part of my body. For hygienic and safety reasons, I recommend you to have underwear.
Hygienic: after going to toilet, some urine can remain in the belt. Having underwear and a hygienic pad avoids bad smells.
Safety: a belt and bra at made out of metal. They can destroy your clothes (it happened on my beloved red shirt, destroyed by padlocks in my back). A fabric bra under your clothes protects them.