Re: Spot the difference...
I would keep the guard cover!! AFAIK it isn't bolted to the engine and it's made of platic so yes it will provide some protection and shouldn't cause more problems. Mine hasn't got one but I intend getting one for it (Only had it a week).
The effect of the strut brace will depend on your driving. The strut brace helps to prevent body flex between the suspension towers and helps to keep the contact surface of the tyre flatter to the road during hard cornering. Obviously this is more noticable on cars with a flimsy body shell that flexes a lot, like a citroen AX or similar. If you like to corner hard then it is worth while and you don't need to lower your car, though I have heard that the lower brace makes the biggest difference.
Why don't they fit them as standard? Because the average driver won't notice any difference, but someone who drives a car hard will. Also why spend an extra £50 on each car when you can sell them to the customer later and get paid to fit them too!! If they block access to parts of the engine it could also increase servicing costs as labour times increase and this isn't what company cars owners want!!