I personally think nappy bags are unnecessary - and I am someone who would easily spend $400 on one if I thought it was worth it. Honestly, a nappy roll is all I've ever used for my 3 kids. I find nappy bags either 2 big and bulky for one baby or not big enough when you have 2 little ones in nappies etc. And I could never find anything in it.
I just use my normal handbag and bought a cloth bag for the nappy roll which also easily fitted in spare clothes, food, bottles - whatever you need.