In short, Microsoft Office Compatibility.
I've encountered too many compatibility issues with people using non standard Office Programs. (i.e. OpenOffice)
Kees_B, the weblink seems to recommend Office HD and Polaris for retaining Office Compatibility.
Quote: "When you do need to preserve full Office compatibility, Office HD is your best choice, as long as you don't need to import documents containing charts. If creating (or preserving) charts is essential, I'd give the nod to Polaris Office."