i would imagine as others have said earlier that the drain grommet is in effect a one way valve with a flap to allow water out and hopefully no fumes back in. it was originally designed and fitted by gm and if it didnt need it they would have saved on production costs .theoretically there shouldnt be any fumes coming from the engine but as we all know these engines have a history of various oil leakage areas and after sorting them there are some hard to reach areas to clean off. so personally i would refit the cable grommet and clean up the drain grommet and refit .the valve realy needs checking regularly for blocking and cleaning. incidently when checking for oil leakage /smells on the passenger side its noted that the oil filler cap extention tube can leak on its bottom o ring and also from the cap especially if the o rings have hardened over time.
also a tip on a BASIC check of the breathers to check they are at least partly open and not totally blocked is to get a piece of hose that will just fit onto the dipstick tube end and blow into it.if they are totally blocked pressure will build up and be very obvious but if there is a way through you will hear yourself blowing into the plastic ecotec bridge.a quick basic test if you are about to set off on a journey and dont want or have time to get dirty. i check mine this way when i check oil level and clean breathers out properly at oil change time
Thanks, cam in head, for your comprehensive advice. I performed your breather function test, all well there, and no hissing from cam cover gasket region. As shown in the pic, the grommet taking the wiring through the bulkhead was displaced, good thing too, because the scuttle drain grommet was completely blocked with mud. I failed to relocate the wiring grommet, so pushed the drain grommet downwards out of the scuttle bottom plate, hoping I could retrieve it from below; not so, there is a heat shield in the way. I tried to pull it upwards, but failed.
At least the scuttle will drain properly now. I suppose this is the state of affairs I should expect on an 18 year old 200,000 mile car. At least it has not flooded the cabin.