Since amazoness monsters are basically low-level and their ATK is not high, the card is useful to remove high level monsters your opponent controls. Amazoness Fighter- no any battle damage you would take, while Amazoness Swords Woman throws the damage back at your opponent. Equip to Amazoness Sage for spell/trap destruction in addition to monster destruction.
This is still worth using, however. The protection makes Onslaught much more dangerous. (Which is saying a lot since it's already a great card.) Keep in mind that Heirloom's effect being mandatory means Onslaught's effect can be chained to it, so it won't interfere with Onslaught banishing things. This also makes Swords Woman and Sage much bigger threats since you can just ram them into things.