You are missing the point, and not reading the things I quoted from the help files. And taking my posts out of context...
The first setting says that ANY executed program is scanned, unchecking it, will scan NONE.
The second one is a preventative measure of sorts, this setting will as it says, scan ALL files created or modified...
Having the second one on and not the first, would in theory be an option, however what about files that are not created/modified?
Say from a flash drive or cd, they are executed (not modified/created) and therefore would not be scanned...leaving you prone to infection...
Definitive enough?
In the end, it boils down to user configurability, avast! is designed in a way so that it is as configurable as possible...so it is your choice...