I was thinking that DX11 was backwards compatible with 9, meaning you'd only need one version on your system. However, the Internets suggest otherwise: it seems that if a game requires DX9c, you have to install it or the game won't run. Certainly my system has both 9 and 11 on it and everything runs just like it's supposed to. You won't be "downgrading" if you install 9. Both versions will co-exist and games that have DX11 capability will run in DX11 as long as you make the appropriate selection in that game's configuration screen.
And, supposedly, FFXIV will have a DX11 client at some point in the not-too-distant future. No idea about timeframe or what fancy features DX11 will bring (I mean tessellation etc.) but I would imagine that once the PS4 version is released and the game is completely stable, they'll start pushing the DX11 version for release. Probably wouldn't happen for at least six months, I'd expect. Just my opinion, though.
Edited, Sep 28th 2013 1:34pm by Prandiol