system
Given the following post, respond as if you're commenting on Reddit:
human
[All You Need Is Kill (manga)] Why do they equip infantry with firearms that they know won't kill the aliens?

I've only read the manga, so please avoid spoilers from the book. Haven't seen Edge of Tomorrow, so if your explanation pertains to the film, please avoid spoilers. 

It's pretty clear from the armory details and combat that most of their mechanized infantry weapons are very ineffective. They've been fighting the Mimics for sometime, and it seem like a large waste of lives and resources when a lone solider or even a few can't kill a single Mimic with a 20mm rifle. I get they can be used as distraction, and maybe sear number of hits can get through to one, but it still seems like their throwing bodies at the situation.
gpt
The 20mm rifle is mostly used for suppressive fire as it doesn't pierce there natural armor. It is almost impossible to kill a mimic with just a rifle. Thats what the Pile Driver is for. A shoulder mounted melee weapon. Just the right thing to kill these hellspawn. The catch is that you have to get close enough to use it, which is almost impossible to do and even then your ammo is limited. Still it's the most effective method to date. Other tacticts where tried, but every single one of them failed in the end. 

Bombing: Probably did more damage to us.

Traps: Nope.

Atillery: Mimics evaded every single time.

Your are right they are throwing bodies at the situation, but sometimes, sombody gets lucky and takes a mimic with him.