There's not much of a reason to implement it if it doesn't add to the experience that Bungie is planning to create with Destiny, and if it is going to get outclassed by more dominant strategies and is going to be largely ignored by the community (they can figure this out via playtesting), then why include it in? If it's not making the game more deep, it's making the game more complex, which isn't a very good thing.

Understandable, but considering there's, essentially, a sniper/stealth class (or so it seems), then I think the prone ability is very likely to make the game more deep, especially when it would just be a quick double-click of the crouch button.

But in my mind, you can create a nice stealth experience through crouching without needing to go prone. Prone just feels so clunky, and designing environments that give you nice cover while you are prone rather than crouched seems asinine. In Halo and in stealth games like Splinter Cell or Dishonored, I've always been happy with the crouching mechanic, never have I wanted to go prone. It could happen, I just don't think it will.