Let me try to clarify another point:
1. The mail shield if not in use, uses no resources, so not really an issue.
2. Just because you only use webmail, doesn't mean the mail shield is redundant.
3. The mail shield could be your first indication that you have a hidden/undetected spambot (malware) on your system.
4. Some malware comes with its own basic smtp function, so it doesn't need yours (if you have one installed or not).
So the Mail Shield essentially has an alternative, undocumented function, I would say keep it and not only that but bump up its Sensitivity.