When Microsoft says OOXML is good because it promotes choice for users, they are telling a fallacy. They are transfering to the standard a role that should be on the product side.
The OOXML standard versus the Microsoft Office product.
Everybody knows that to have many products that use the same standard is the right thing for the standard and for the users.
GNU/Linux (not just the kernel) positioned as an OS standard and as a brand gets weaker when many different distributions package it in different ways and flavors. The result is that the mindset of what GNU/Linux is is not a consistent concept across the industry in general.
Of course in the case of OOXML Microsoft is pushing it in an artificial way, and on the Linux scenario to have many distributions is a natural process, but just think about it.