Any screen is better than none at all. I went from a naked B4 to one with a National Cycle F-15:
and it was night and day. I now have a Plexistar II:
(which is probably bigger than most people would like to have on their B4s) and can easily ride in winter with excellent shielding on the head, torso, and (most importantly for me) hands.
Fortunately the Plexistar comes off a lot easier than the F-15, although I leave it on even in summer 'cause it comes in handy if you get caught in the rain.
But the best thing about even a flyscreen is that it smooths out the ride. I used to get my ass kicked going over some bridges in my area with winds buffeting and blowing from all directions. Once the F-15/Plexistar went on, it was smooth as silk.