If the virus in question is being written to file and then executed - Standard shield will suffice. Otherwise not.
And IF the security level is let on HIGH, otherwise even the Standard Shield won't detect some infections.
I see not a reason to use only the Standard as dedicated providers will catch the virus first and better sometimes.
Web Shield, for instance, HTTP (web) traffic
before it reaches the browser.
Web > Web Shield > Browser > Standard Shield
All data is first checked and then passed to the browser, and if the data is cached it can be also checked by Standard Shield.
So there is much smaller chance of getting infected by some exploit if the data is scanned before it actually hits the browser itself. In other words, the idea of the web shield is to scan the http stream, to detect any possible virus infection before it has time to get established on the local disk.
Besides this, avast is the first antivirus with a dedicated NNTP scanner for "News".