Mass-effect – How to unlock the M55-Argus Rifle as a pre-order on Mass Effect 3

You can learn a single power in the Normandy Medical bay. Learning a new one will replace the old power. It costs 5000 credits.

Powers will be unlocked through dialogue with your companions. Each has 2 powers to give, one generally unlocking fairly early and the other one significantly later. Make sure to talk to them after a mission both on the Normandy and in the various Citadel districts.

If you're an Infiltrator or Soldier you get a "time dilation" effect with the sniper rifle which slows down time. Soldiers have to use the Adrenaline Rush ability to slow down time which can allow you to take a careful headshot or two, Infiltrators actually get a passive slowdown every time when you're scoping with a Sniper Rifle.

The slowed time allows you to take headshots easier, in addition these classes have a high weight allowance, so carrying around Sniper Rifles isn't as detremental to your cooldown.

Infiltrators get additional benefits for Sniper Rifles, as their Operational Mastery skill increases the sniper damage and time dilation effects at certain levels. Tactical Cloak also allows you to take your time to get a good headshot while dealing extra damage to boot.

You'll never really be at extreme range, but the extra damage a sniper does comes in handy if you can headshot a humanoid foe. If you're not a good shot or you're not playing soldier or infiltrator you probably shouldn't bother with Sniper Rifles because of the weight issue you note.