A couple people have told me that they see no point in some of the weapons secondary and tertiary functions, but I disagree, here are a few of the most common ones:

Jackal(standard sniper rifle)- In multiplayer, the radar jamming function can actually be a very important squad based tool, but only against other humans. If you can get a sniper in a good vantage point with one of these, you can jam the enemies radar, set up and ambush and the enemy will not be able to see where you are shooting at them from allowing team members to take less hits and fight longer.

Magsec 4(futuristic silver pistol with scope)- you can exploit the ricochet function to hit an enemy twice, if you aim at the ground between an enemy players feet, you get the bullet to ricochet off the ground at the same time it hits them, doing double damage.

Hawk(boomerang blade)-if your enemy is on an uphill slope from you, you can get a hit that will completely take off all armor and nearly kill them. All you have to do is aim at the ground between their feet, as with the magsec 4, and the hawk will "hug" the hillside, hitting your enemy multiple times.

Thats all I have time for today, I will list more later!

On The Map Desert, bases 3+4, When Your in the middle there are these big rocks, get a jetpack and a sniper rifle and go up there, either hop out or stay in the jetpack and shoot the enemies.

On the mission were you and jack escape it tells you to take out the hover crafts using the demo kit...DONT! It takes ages, just lock it with your locktopus or shoot the hell out of them!
Peace out!

If you are out of ammo and there's an enemy near by then if you sneak up behind them you can kill them in one hit if you get them in the back of the neck.