Antivirus mail scanning

Why does it exist? I have seen it cause problems with every mail program from Outlook Express to Thunderbird. Every error any mail program I have ever seen have, can and has been caused by AV mail scanning. This isn't any one particular AV either. I've seen it on everything from Norton to AVG. From the research I've done, mail scanning is completely redundant anyway so why does it exist? At least once every day, I have a customer telling me the problem can't be with their AV because they disabled it. Well, guess what? If you don't turn off the mail scanning feature, it still causes problems whether the AV is disabled or not. The only thing I can figure is that these AV manufacturers throw it in there to make the computer illiterate feel safer. What do you folks think?

Sorry about this little rant. I'm at work and haven't had a call in over 3 hours so I'm bored. =P