Well, according to avast!, Network Shield 'monitors all network activity and blocks any threats that are detected on the network. It also blocks access to known malicious websites based on the avast! database of infected URLs'.
Furthermore, let's make a test: For a known malisious website, when enabling Web Shield, it absorbs the whole webpage, and the notification is as 4.png
When enabling Network Shiled only, the webpage was displayed successfully, infected elements were blocked selectively (well, I'm also confused... Since the notification says 'malisious URL blocked', does it only check url agasinst its database?), as shown in 2.png and 3.png; when the infected script tried to write in Firefox's cache, it was picked out by Filesystem Shield (1.png).
So since the infected elements would anyway be kept from the computer, does it make any difference between enabling and diasbling Web Shield?