I think there is too much fuzz about this topic. What we have seen so far shows that we will have iron sights were it makes sense to have them, which is a nice balance betweent Fallout 3 and New Vegas. Fallout 3 had no iron sights at all, and New Vegas put the on every weapon (I do not know about the big guns though, never played them). I want to make sure that by iron sights I mean aiming down the barrel and have a visor of some sort, either iron sights or any other sight.
There are practical reasons for just zooming in on some weapons instead of aiming down the barrel. The big guns are an abvious choice. In order to aim down a barrel you have tro bring the weapon to your shoulder, you can not do that with a gatling. You could expand that to the missile launcher as well, though they usually have some sort of locking mechanism or whatever.
This brings me to the next argument for not having them. While the general art design and weapon modelling is great, in order to clearly distinguish different weapon types (conventional, energy, big guns) you have to make them look different, that is why the energy weapons are bulkier, just look at the plasma rifle in comparison to the standard hunting rifle. Converntional weapons are slim. Energy weapons really big. Now aiming down the barrel or with iron sights means one thing: The barrel obstructs your view. And by quite a large amount, too. Launch New Vegas and use a plasma rifle, then you know what I mean. That is the main reason why the laser rifle and the plasma rifle do not appear to have iron sights. A reflex sight or a scope however offsets this disadvantage, because the barrel is taken out of your FOV.
So iron sight, will be there where it makes sense, meaning conventional weapons and maybe small frame weapons as well, but not on weapons that are bulky and view obstructing. So you will have the same hipfire zoom we had for quite a while. This of course is only until you get any other sight by mod.