Snipers rock. They intimidate your opponent into avoiding open areas, and pin down wounded units. They do good work against missile tanks especially if they're not in view of the enemy. They're highly mobile (like all the Marines) and can be where your opponent didn't think they'd be. They can take down almost any Marine in one hit and murder XTs at long range.
But they die quickly in close combat, or against fast-moving enemies. They die quickly against buzzers, hunters, drones, splatters, AIs, XTs, commanders... never use a sniper in close.
The biggest key to keeping Snipers alive is the have them behind cover from the front, using diagonal fire lanes to opposite sides of the map, and having a couple of Spotters up front triggering your opponent's OppFires. Most players will have their Snipers aiming directly forward, so you'll avoid Sniper Duels this way.
Snipers are far more useful for the pinning effect, as lots of players just won't walk out in an area where there is even the possibility of a sniper.
Snipers can unreliably destroy walls and objects, and very reliably destroy faraway doors.
They're great for keeping track of the enemy without being detected, or spotting from far enough away that you're not really in danger.
I use this a lot! Cut holes in the front of buildings or creating fire lanes. You can prepare an area for your troops before they approach. Putting holes in buildings also creates a nasty surprise when the enemy enters the building and is instantly the target of Sniper fire. He won't see the hole until it's too late. My Snipers spend a LOT of time improving terrain during the game.
In the mid to end game, I'll position my Snipers to control fire lanes. I prefer to set up diagonal or side to side lanes instead of the "expected" front to back lanes because:
Enemy Snipers rarely cover these lanes.
Viewing diagonally or across the battlefield greatly increases your spotting ability.
You can target units that your opponent considers "safe" from enemy fire. This is especially true if your Snipers are positioned on one edge of the map and are aiming across it.
By firing diagonally across the battlefield, you can engage front line troops at long range where you have an accuracy advantage.
When attacking or defending, I'll position my Snipers diagonally and well behind my front line. Their fields of fire cross through my front line troops and I never leave them exposed at the end of the turn. They are not exposed to the front at all. What this does is makes them almost impossible to target since my dancing Grunts and other troop movements attract the enemy's OpFire commands.
Used like this, they can pick off targets at their leisure with a great amount of safety.
A great way to kill barricaded troops like that cornerdancing Grunt half way across the map. Say the Grunt is ducking in and out behind a section of wall.
Plot a Terrain Fire at the wall followed by three Direct Fires, another Terrain Fire, two Directs, etc. Your Sniper will not take down the wall section and target the Grunt who's now exposed during his entire movement. If the Grunt is unexposed at the start of the turn, use Terrain Fires and OpFires instead.
Another example is when you suspect a blank wall is hiding the enemy.
Put the Sniper on Halt-On-Sighting and Fire-At-Will, plot four Terrain Fires at the wall(s) and then a Couple of OppFires. Now your Sniper will take down the wall and engage any enemies he finds behind it. (The OppFires ensure that he engages any enemies that were spotted at the beginning of the turn if the room ends up being empty.)
See Assembling A Fire Team for more detail about Sniper Fire Teams.
Thanks to Crank
As far as having a Grunt stepping out and drawing fire while a Sniper steps out immediately behind him and shoots, that was my preferred way to remove Snipers. I'd use a Sniper or a Commander as the shooter. In an ideal world I'd have the enemy Sniper spotted during the plotting phase so that I could Red-Line him.
The Grunt would step out first and run laterally across the Sniper's fire lane. 1/4 second later My Sniper would step out and fire. I would try to get the Grunt back behind cover by 2.5 seconds, but I I couldn't, I'd run him laterally until he reached cover. The combination of his Lateral speed and the enemy Sniper's small firing cone would usually keep him safe.
Thanks to Crank