Why? Well for one, it would show that the company actually cares about doing more than the bare minimum; I mean, this is an area of the game overripe for expansion.
For two, unless I'm the DM, most of the people I've played with seem to be allergic to 3PP; I can somewhat understand the sentiment, having endured the "d20 plague" when everyone and their cousin was cranking out products under the OGL of often dubious quality (granted, WotC wasn't immune to cranking out stinkers either, but at least their products got some use, unlike the stack of dusty books I have in storage).