The Mail Shield isn't going to be using bandwidth or resources if you aren't using normal email, e.g. you are using webmail, viewed through your browser as the Web Shield would be monitoring that.
I would not only recommend that you leave the Mail Shield installed, but that you increase its Sensitivity to High Heuristics. The Mail Shield could give you the first indication of a hidden or undetected spambot on your system.
Many of the settings that you mention will be dormant (not using resources) if there isn't any activity that it is monitoring.
That would also be correct if you aren't using Home Network, whilst I have two computers they aren't networked so I don't have the Home Network option installed.