Offset iron sights like that are much,
much faster on close range targets than using a magnified optic, and obviously the magnified optic is going to be much more effective for engaging targets at distances out to 500+ yards.
The biggest disadvantage with offset irons, however, is that they can snag on stuff. That said, such a setup is still quite popular among Tactical or Limited Division 3Gun competitors, and will likely continue to be widely adopted until the new scope designs that go from 1x-8x come online in a big way.
Currently, these are the configurations that a lot of Tactical/Limited Division shooters use for their rifles:
-Fixed magnification optic with offset irons as pictured.
-Variable 1-4x scope such as a Schmidt & Bender Short Dot.
-Fixed magnification optic with an occluded eye scope cover. (This is what I run.)
Try going prone with that mag sticking out a foot or so and you'll see the reason for a side mount sight.
I can and have gone prone with an extended mag in my AR, and while it looks awkward, it's not the herculean task so many people seem to think it is.
An AR-15 can easily be deployed to 500 yards with iron sights, let alone with some type of optic (my gun wears an Eotech).
AR's can and are effectively deployed with iron sights out to 600 yards. High Power shooters do this regularly. However, in my experience and observations, when shooting under field conditions to medium and long distance a magnified optic is much, much more effective, allowing the shooter a higher probability of first round hits, as well as making it easier to make corrections for follow-up shots.