By doing that, they can prevent you and everyone else from using any competing product, including this one.
I will say it again: if your document breaks it is not the fault of Quip or whatever, it is a feature of using Microsoft formats, and Microsoft revenue comes from that feature.
If you want to compete with Word, make it righteous and compatible, and I will purchase a copy immediately.
However, given the choice between the moral high ground or a text editor that does what I need it to do (including compatibility), I will choose the latter every last time.
You not distributing closed formats (that's why i don't use M$ office, apple products or buy sony weird media that came out every now and then) is the right thing
You preventing yourself from accessing those formats are not harming them, just yourself.