Know, I know the next relates to carp, rather than barbel...but as the question has been asked, I will explain why I used a net screen in the past
If you are sat in a bivvy, which the features of the surroundings dictate is only three or four feet from your bait in the margins, in a lake with wary old carp that KNOW when a sparrow farts....then your ideas of what is or is not worth trying undergo a sudden change. I quite agree that fish are quickly alarmed by clumsy footfalls, bankside noise etc, etc....but once you have eliminated all that obvious stuff, and the fish are still spooky...what do you try then?
You sit there, knowing the fish can very likely see you...in fact the bow waves heading up the lake if you make any sudden movement prove the point...so you end up too paranoid to move...to pick up your cuppa, scratch yer bum...anything! THEN you appreciate the value of a small barrier of camou netting between you and the fish

You still have to be ultra carefull...but at least you can blink
I doubt your average barbel are this wary...but who knows? Results from under the rod tip stuff might very well improve if more precautions were taken.
Would it matter if this screen were black instead of camou? I guess that depends on how many black bushes and trees you have on the banks of your water...fish are NOT as stupid as many think
Cheers, Dave.