What responsibility to stand against proprietary formats do we have? That was never my goal when becoming a developer. I want to make simple and useful tools for people, and a .xls file is the most simple and useful format when the goal is to edit data in Excel (which is the goal 99% of the time when you're not dealing with another developer).

Of course open formats are best, but sometimes they're not a good option.

Our responsibility as developers is to deliver a working solution within the parameters that we are given, not to push an agenda (no matter how strongly we may feel about it).

Like it or not, most of the businesses in the developed world are still using MS and the associated file formats, and will be for quite a while yet.