While common on modified cars, an Air-Oil Separator (AOS) can still be beneficial. Even in stock form, oil can still make it's way through the intake system. Boxer engines, or horizontally opposed engines, have pistons that move side to side. When running, the G-Forces push excess oil to the tops of the pistons. Because oil is just as combustible as fuel, this causes irregularities with the controlled explosions in the engine, subsequently creating multiple explosions or "detonation". This can lead to causing the ever-popular "ringland failure" problem. Installing an AOS eliminates the oil vapors in an intake system thus eliminating the loss of octane. Without oil being burned off and the octane levels up, the engine can make more power!
On a stock turbocharged boxer engine, that oil getting through the intercooler doesn't make its way back to the engine. As it is subsequently burned off, your oil levels start to drop a little. It's not a significant amount, but over a long period of time it can be detrimental to the piston rings if the engine oil level isn't kept up. Tie in the fact that the detonation and oil buildup in the engine are already wreaking havoc on those same rings and it's an accident waiting to happen. Again, this can still happen on a bone-stock WRX, so even those who have been careful with their cars run the risk.