An AP close combat weapon and CC skill of 19 mean the Pretas can take out line troopers, but they will have to get lucky to take down any one skilled in martial arts. However, the real reason you are taking is their chain rifle. Along with Climbing Plus they can get some weird angles or reach the top of sniper towers with ease.
PH 14 is all the Gakis have the keep themselves safe. Should that fail, they have dogged to help assure you get your investment in them back. Remember that since they are impetuous, they won't even get partial cover unless in a link team.
Pretas can fire their chain rifle through smoke (deployed by the Oznat, as an intuitive attack long skill. They will have to rely on their WIP of 12, but their is little chance of enemy reprisal making this a good option against enemies with defensive modifiers or who are likely to gun your down in ARO. Pretas are irregular, and likely to die in the impetuous phase, but keep in mind that an unconscious irregular model retains its order. You can still turn it into a regular order with a command token even if the preta is unconscious.
Pretas go down to any fire. If you have to wait for them to come to you, set up mines or put models with Shock ammo in a place to ARO them. Shock will prevent them from using dogged. If you have to take them out in assault, use your knife, since it has Shock ammo.
Pretas are cheap enough to bring in almost any generic combined force. 4 of them together can make a very dangerous addition to a second combat group. You are likely to lose them, but rolling the dice on getting one through to the enemy backlike is worth it. If you want throwaway living bomb, consider their less fertile sisters the Gakis instead. In the Morat Aggression forces Pretas can only be purchased by purchasing a Oznat. The Oznat also allows them to be linked along with Gakis which can make for a very cheap group of orders to power a harris or other elite Morat models. The link also solves their impetuous problem, and lets them get burst 2 on their chain rifles, so it can make a powerful close assault unit.
Keep in mind the Pretas' extreme impetuous when deploying them. They will be running a minimum of 6" at the enemy every turn, so try to keep from losing them to a free ARO.
Active Turn Role
The Pretas want to get into as large a cluster of enemies as possible and fire their chain rifle. If being brought as part of a link, they will either hang back, or try to stay out of sight as the link moves up to put themselves or the Oznat to use at close range.
Dodge is the best option for the Pretas most of the time. Using their chain rifle will have to be a judgement call. Most of the time it is worth it to potentially wound an enemy model, but sometimes it is worth keeping them alive until the active turn when you can move into a better fire position. they will dodge at 14 and have Kinematica one, allowing them to get to safety with ease. Keep in mind that Kinematica also increases their engage range, and they should always try to engage against a foe foolish enough to walk near them.