You are right that .rar files are inactive and that the File Shield will spring into action once opened and that it is safe until the infected .rar file is opened. I think that was also a topic in Avast facebook fanpage or was it the Avast forums...or was it Wilders'... Shared my experience so the probable Avast user will know some information same as all. This will help him in his selection in addition to his trial. Experienced that late 2011 when I was with AvastPro ver5+. Downloaded the same file from the same site with another pc I have with Avira/MBAM and both stopped it cold during download.
Though the files .rar files are inactive/safe I prefer during the download that Avast will detect it so the download will be cut/stopped. It is inconvenient for the user if after a long/large download, say, a .rar/packed file, say, a repacked- WAIK (which is a 1gb file) downloaded from a site other than Microsoft but a site from a notable forum, the user will be surprised/dismayed that what he downloaded is infected. Time/resources was lost.
Anyway that's only an example and of course good/safe surfing habits is king as well as common sense. As stated I use AIS v6 in a couple of my systems still. Feature rich and popular if I might add. Personally I do not use the Behavioral Shield/Auto-Sandbox/Sandbox as I have SBIE and CIS with D+ in one and Behavioral Shield/Auto-Sandbox/Sandbox/WebShield in the other one with PrevxSOL/Online Armor Premium.
Tested MSE twice last year and well did not like it. Though straight to the point I believe it lacks power and need be in addition to a layered set-up and not just a standalone app. Don't know about the latest version though... If having to choose MSE over a freeware like CIS v5 suite, I'd choose Comodo hands-down. But then again that's just me.