Re: 156 Under Tray
They are almost certainly to help the car get through it's type approval noise test. That's what they are used for on other cars that I've been involved with the type approbval on. They just prevent some of the mechanical "thrash" noise reflecting off the tarmac and getting to the noise meter. You won't notice any difference inside the car. I don't think they're to prevent oil spills. If you have an oil leak, the tray will only postpone the oil getting on to the road, not stop it! Besides, if you have an oil leak, there is somethign wrong with your car and it needs to be fixed!
I've removed (most) of the tray on my wife's car (same reason as everyone else - kept grounding) and noticed no difference in aerodynamic stability, performance or engine cooling whatsoever. The bottom half of the engine bay IS a bit dirtier though.