Since “On Access Scanner” will immediately and automatically scan any files and programs when they are opened, created, saved, copied, run and executed, so the computer should be absolutely safe.
Then why is it necessary to schedule regular scans ?
A lot of the newer malware variants will infect system restore as well as other system files, and on access will not be able to access these files to scan them as they are protected by Windows and are not executed until needed. A full scan will scan these files regardless if they are executed or not and will tell you your system restore files are infected and allow you to remove them by disabling system restore (temporarily of course).
This is only one example of many.