Far Cry 3 attachable weapon sights are inferior to the standard iron sights.

Examples for comparison: http://imgur.com/a/ls3Lp#0

The attachable weapon sights, while giving stats like accuracy, range and mobility, are far inferior to the traditional iron sights in terms of visibility and field of view. The tip of the cross-hair wider and there's also a lot of lens flare if the sun is behind you.

While the Marksman and Optical sight add a bit of zoom, they also make the weapon less effective in close range due to a narrower FoV. Furthermore, the advantages of the zoom are completely negated by the blurry cross-hair. And for long range you are better off using a sniper anyway.