Personal experience is one of the greatest assets of an internet community - thank you for sharing yours - but in the complex world of security it's really not enough, since no one individual (save for maybe someone on staff with an anti-malware company) can have experienced even a small minority of all the malware that's out there.
The thing the scan does is back up the operation of your shields. That it's different in its operation from the shields is the key. The shields check only certain kinds of files and only small parts of them in an effort to be efficient enough to be tolerable. The scan can be set to read all files and check them in their entirety.
It's not hard to imagine a file written in such a manner as to carry a malware payload in a part that's not scanned by shields, but which can be revealed later by overwriting the front part of the file with a block of no-ops.
Knowing that there's a difference between the capabilities of shields and scans, and depending on what makes you feel comfortable, a daily scan that normally doesn't catch anything could still be well-advised. Not every one of the millions of types of malware is knocking at your door every day!
-Noel (possibly more paranoid than Para-Noid)
P.S., Consider setting this:
