It's our goal to be a good Mac citizen and to provide the best productivity experience on the platform. For the most part, that meshes up easily; other times, we have to make trade-offs. Spelling/grammar is a great example of that. While the OS does provide an acceptable dictionary built into the system, ours is significantly deeper. Imagine if you ran a spellcheck on a document in Word:Mac 2011 and everything came out fine, but then you ran the same document through a spellcheck in Word 2010 for Windows and it came up with errors. To maximise productivity and avoid cross-platform issues like that, we use our own.