I've read a few posts stating that email scanning is unnecessary and redundant when running a resident scanner?
Emails are a very common way of viruses spread. A dedicated provider is, for sure, safer than just the standard shield as the messages are analised by themselves and not only like 'a file in the disk'. Only at 'HIGH' sensitivity level of protection you could 'try' to work without the email provider.
Anyway, why do you want to do this?
I've also read that sometimes they can mess up an inbox or messages. Any truth or thoughts on that?
Trying to delete an email message (in some email clients, e.g., Thunderbird) could let to entire inbox deletion.
I doubt it won't be the same if you try to 'caught' and delete the virus using the Standard Shield, more, I think it will, for sure, delete the entire mailbox.
Again, I don't see advantages on not using the email provider.