In Crysis 1, it was imperative to be able to modify, because you needed to engage targets at different ranges, perhaps cause distractions or not be seen by your laser sight. This was also because the objectives were stationary, so you knew your targets were inside and you had some leeway with time to engage them. Now, since we have no vehicles and maps are small as heck, modifying weapons seems fruitless. Sure it gives us some "freedom", but I don't see what good it will do, as it hardly makes a difference what scope I put on a gun if everyone is running around and the person I was about to take out has already changed position 3 times and more likely than not killed me while I was modifying.
My main point is that, for weapon modification to be available mid-match, there has to be a point to it, namely a mode that features large maps, large match times, most likely vehicles, and some objectives/buildings. Otherwise you have the option to change your gun in the middle of a match where it's pointless because you're all fish in a barrel, and changing your weapon is just a waste of time.
-Zanthir