Is it worth it? Well that all depends mate... Some will say that you don't need an undertray, some will say you do.
My opinion for what its worth is: if it was originally fitted, then it was fitted for a reason...
I don't know what the climate is like in NSW but here in the UK we get a lot of bad weather and that is usually in the form of rain and snow eventually.
Our roads are gritted when the snow comes with rock salt
Rock salt can strip paint and attack bare metal like a 5 year old can demolish a pile of candy.
I have seen Alfa's with no undertrays suffer from bad corrosion on the legs that hold the front bumpers on and also bad corrosion on the radiator bottom and cross member.
I have also seen Alfa's of similar age and mileage with the undertrays correctly fitted without this type of corrosion.
I can only draw one conclussion why.
Most owners fit the alloy type to prevent scraping over speed bumps... Another wonderful British idea
As the alloy has a deflective angled bend in it to prevent this. Worth it if you live around speed bumps and are considering lowering the car.
The question was is it worth it? Simply compare the price with a new plastic undertray and deem it nessessary if your roads are as bad as ours , then you have your answer