It is extremely frustrating to have something sent to you as a MAC user that you cannot open because the product involve is Windows only. Sure you want cross platform interoperability if by platform you mean operating system.

There will never be a computer or OS that 100% of everyone uses. There is always a 10-30% market product that keeps the big guy honest.

I have had the frustration of sending a file to a group of 15 people to do some cooperative work upon only to find that 2 cannot open it. This kills the whole process. You have to fax it to them or get them to buy the software, or ... .

Intraoperability is based on a hub and spokes model where one player or component is given a central role. OOXML is designed around this model.

OOXML is about allowing other people to interact with files that are primarily generated and manipulated with Microsoft Office. It is not about full interoperability, which would enable competition with Microsoft Office, for obvious reason.

In this context, although it'd still be better if it were platform independent, whether it is or not may not be as important. No matter what, you are subjected to a set of constraints imposed by the hub.

When interoperability is sought, then yes, platform independence certainly adds value.