The kroot has three strengths: They can infiltrate, get good cover in forests, and they are cheap. Remember that infiltrators also can outflank. Outflanking kroot can cause trouble for the enemy, and may even take out a vehicle if shooting at it's rear, or by assaulting it. They will not live long, but don't worry, they are cheap, remember? :-)
Another way to use them is to let them dig in somewhere, preferably in a forest (maybe it's time to get some model trees?;-). From there they can harass the enemy and draw some fire, maybe even letting them go to ground to make them last longer. Watch out for flamers though, or they will easily get, well, toast... But don't worry, they are cheap, remember?
There is always that exception that confirms the rule, right? Above you can see two squads of kroot advancing on a unit of terminators. The whole thing ended up in a melee, and the kroot won...