Probably going to stir up the pot here, but a "lively" discussion is always good.
While email protection is nice to have, it's not essential. What IS essential is blocking a virus when opening a file, which is what real time scanners do.
It's like a bad guy approaching your house. Email protection will stop him as he comes up the driveway. Real time scanners will stop him when he gets to the door as you open it. Either way he gets stopped, only with one way he gets stopped earlier. Now some might prefer that "pre-emptive strike" earlier, but as I just said, BOTH ways stop the bad guy.
As long as your antivirus will alert when you try to open an infected file (an email attachment in this case), you are protected.
Now here's the arguable part. Many people believe that opening an HTML email alone will result in infection. But doing that has never exposed anyone to a virus UNLESS you open the attachment. If doing that gave you a virus, then simply visiting an HTML web page would do the same. If you visit a web page that harbors viruses, you HAVE TO download something or click on a disguised link that has virus code. Just visiting the web page alone will not give you a virus.
Anyway, as I said email protection is nice to have but it's not essential as long as you're protected when opening attachments.