When game is in development, then "more" simply slows down development. Every time you change something in weapon mechanics, you will have to fix/adjust every singe weapon. Every time you will make some change in movement, you may have to make adjustments on most maps. On prototype maps it can be done much easier.
Adding weapons/attachments at this moment is simply waste of time, since i plan to change model of arms and hire someone who will make good animations for weapons. There is no difference in gameplay between prototype and non-prototype maps. Prototype maps are easier/much faster to make and performance on these maps is much better. Many players play this game on "calculators" and i am not joking here.
And if you are not familiar with level design, then prototyping is part of map design. You don't create detailed maps from very beginning. At first you create map blockout and you test gameplay.