front under car bib thing/crappy stone shield thingy = AIR DEFLECTOR
.. It is (should be) held on by ten screws. They come loose after a bit and fall out (the more you bash it and/or remove/refit it the quicker this occurs). Replace any missing screws and put some loctite nut-lock on all of 'em and put wide washers under any where the bolt hole in the deflector is a bit worn. This should keep it nice and secure. Where the front edge of it meets the underside of front air dam, it's not a very close/even fit - duck-taping along this join is a good thing and stops air pressure getting in and trying to rip the deflector off.
I'm not 100% sure about it's purpose(s) but I believe it's main one is (as the name implies) to deflect air under the car (caused by the lip on it which hangs down) and creates a low pressure zone under the engine bay which draws hot air out of the engine bay from underneath while you are driving and also helps promote air flow through the radiator and engine bay and out through the bottom. Given engine bay temps and lack of bonnet ventillation etc. (and hot air's tendancy to rise if left to it's own devices) I'd say it's a worthwhile thing to keep securely on your car. Modern cars have very high engine bay temps are are designed mostly now to vent the engine bay from underneath like this. It's pretty important IMHO.
- Just an little 'side-note' there for ya's.
Steve.