Let's get straight to the point. Weapon attachments are useless and are waste a slot. Only thing that can be worth slotting is only laser pointer.

1.Make that attachments use own slots.
Slots are precious. Should be used only to enhance weapon characteristics. Attachments are installed on the weapon, not replacing parts, so they should use their own slots

2.Some attachments could use a rework so they could be worth using
Proposals here:
-Flashlight: there are no areas in game dark enough to use this. Also there is Halogen Flashlight tool. How to make flashlight worth using then? Simple: make it reveal stealthed units. As long as you flash at stealthed person, it appear less transparent. It might stun PvE mobs when flashed at very close range as well.
-Silencer: Very limited usage, for weapons at very long range. But from that range, you wont hear sound anyway. How to make it worth using? Make silencer reduce damage and aiming speed (even if in real life it actually increase bullet velocity), but allow low-tech weapons to shoot while stealthed. Might be controversial, but if we have flashlight attachment stated above, it should not be much a problem. Also silencer installed on Sniper Rifle should not have penalties and increase damage instead.
-Scope: This is ok. But it could increase damage to upper body as "virtual headshots" to be even more worthwhile. Also we need some scope visuals.
-Laser pointer: perfectly fine. Can't be used with upgraded silencer at the same time.