Yes, you are wrong, for many reasons - some of which might be hard to explain
You are right that the scanning of WebShield & FileSystem shield is very similar, but:
1. they have different settings of unpacked archives (which, in case of P2P and IM Shield actually works as an optimization - you can scan specific files (the ones downloaded by P2P or IM programs) with full unpacking, while keeping the rest of the system on default settings, which doesn't slow down your machine)
2. the FileSystem Shield doesn't scan
all the files by default (and if you set it to do so, it will slow down your system a lot) - so it may actually not detect some files downloaded by your browser, even if we forget about archives and unpacking
3. if you have a vulnerable browser (or browser addon/plugin), the exploit / malicious code may be executed there before, or even without, being written to disk - i.e. the damage can be done before the FileSystem Shield has any chance to stop it.4. some detections may be bound to a particular shield; I'm not saying there are many of them, so it's unlikely you'd meet them with 100 samples, but they exist
5. WebShield also acts as a provider of the file source (it can store the information about where the file was downloaded from) - which may be used later even by the FileSystem Shield. So, by disabling the WebShield you actually lower the detection of the FileSystem Shield (or behavior of the AutoSandbox).
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