Enabling DX11 IS optimization for those users who have the hardware that can support it. Stuff like ambient occlusion and cloth effects are able to be rendered with less strain on the system.

Take a case like WoW: Cataclysm, where you can run the game in DX11 mode. It looks exactly the same, but the game takes advantage of DX11 to more easily render certain things and boost your framerate: http://www.tomshardware.com/reviews/...ce,2793-7.html

In what almost seems like a stroke of irony, it looks like the folks most likely to see big speed boosts at the hands of Blizzard's DX11 code path are the ones with fast processors and near-overkill GPUs. If you built your PC with balance in mind, there shouldn't be as much room for API optimizations to augment performance beyond what you're already seeing. With that said, I've been passing this DirectX 11 flag around to a number of friends with lower-end Core i5 and Core 2 Quad configurations, and they're all seeing roughly 20 frame per second performance boosts. So long as your GPU is DirectX 11-capable, there's a fair chance that enabling Blizzard's code path will at least do something for your gaming experience.
It won't help everyone, but it's something.

What confuses me is that one of FF14's main selling points is the pretty graphics. The only games that are designed with DX9 only are old games, or games that don't have graphics as a priority.

Anyway, here is an example of how DX9 is denying the game of features -

A user on the JPN forums asked if we could have capes. This was the dev response (I don't believe it was ever translated here):

Quote Originally Posted by Foxclon View Post
こんにちは。

FFXIVにマントを持ち込めるかどうか、開発チームと確認してきました。
残念ながらマントの実装は難しく、具体的な検討状態にもありませんでした。

やはり、マントを追加するからにはヒラヒラユラユラする必要があると考えていますが、今のFFXIVでヒラヒラユラユラさせるには、必要なスペックをグンと上げてしまうことになり、ちょっと現実的ではないという判断です。
Unfortunately, implementing capes is difficult, and we have no concrete plans for them currently.

Capes would of course need to be fluttering and waving, but to to so would require a large upgrade in the game specs and we decided that it would be impractical.
Quote Originally Posted by Foxclon View Post
昨日の内容にちょっと言葉足らずの部分があったので補足です。

ヒラヒラユラユラするものが不可能です、というような強調した書き方になってしまいましたが、すべてのヒラヒラユラユラが無理ということでなく、例えばスカートは動きますよね。そういうものは大丈夫なんです。

マントで求められる、肩から腰を超えた長い装備を同様にヒラヒラユラユラするには、それを実現するためのモデルデータの都合上、難易度が非常に高いものとなる、ということでした。
Let me clarify what I meant:

It's not that fluttering and waving are impossible, as you can currently see such motion on equipment like skirts.

It's just that capes are large, spanning from the shoulder to the hips, and the model data required for rendering such a large area with fluttering and waving would be too high an increase to be practical.

FYI - it's easier to render simple cloth physics effects on DX11.