Right now Arma 3 doesn't support visible occlusion culling in terms of shape blocking, this can be seen by entering the structure just behind the tower at the air port and turning west (towards the ocean), then turning east (towards the mainland). You will see the FPS drop despite nothing visibly being rendered. This is where Occlusion Culling comes in and could play a vital role to performance increase across the board.

Occlusion culling is a technique used to not render objects in a view space or hidden behind other objects. The Arma series would be the example of picture 2 seen here
http://techpubs.sgi.com/library/dynaweb_docs/0640/SGI_Developer/books/Optimizer_PG/sgi_html/figures/view.frustum.culling.gif
note how the spheres and cone are rendered despite the camera NOT seeing them.

The third image would be what would happen if OC were implimented, they are essentially no longer in the scene and no longer rendered due to being hidden, just as shown here.
https://sslproxy.getsatisfaction.com/sslproxy/SWhAdDNLMG5zdGFuVGlWenmLbJDGd3CABhjZermgcystANA4T6nd0pzV0fSzBRRGOd17A4b8ZLwi6-HESbXI0t5d_RUjeg7VVjYlYJW1m3Esz2caI6KYXGXXwLJwcF9nSz8LN0i99GO3lwueWs0m1A==.png

Saving occluding techniques would be the difference between being the white box here and seeing AND rendering this http://docs.unity3d.com/Documentation/Images/manual/OcclusionCulling-1.jpg
versus seeing only that but rendering all of this http://docs.unity3d.com/Documentation/Images/manual/OcclusionCulling-0.jpg

The absence of this techniques makes it blinding obvious how Arma 2 had difficulty with Cherno, and will have even more difficulty with Ramadi and I suspect will run into the same thing in Altis. In closing, occlusion culling is purely a performance saving technique, it is vital to many developers and would DRASTICLY enhance performance for the Arma series.

It would no doubt take time but consider it a building block, a foundational step rather than "another feature".
The difference would be that if I were in a building in a city I'd only be rendering what I see from the window or the wall versus the whole city behind it.