The big win in using something like Open Office is consistency. The only motives among the developers are to improve the product and maintain compatibility.

With Microsoft products, you get into these games Microsoft plays. They'll nix application compatibility, so that everyone who gets a PC with Windows N+1 has to run Office N+1, thus doubling their upgrade profits. And they tie that to file formats -- they've intentionally broken the .DOC format at least half a dozen times, so that Word N+1 writes files by default that can't be understood by Word N, in an attempt to force updates across the board. In theory, not supposed to happen with the XML formats (either the Open Document formats, or Microsoft's proprietary clones of said formats), but ... does anyone really trust Microsoft not to do that again?