Are Sophos Anti-Virus monthly updates really necessary? Why not just update with IDEs?

Virus identity (IDE) files are not intended as an alternative to monthly updating. They are released as interim protection against new virus threats between versions of Sophos Anti-Virus.

• Each month, the Sophos Anti-Virus update detects up to 1,000 new viruses. Sophos publishes interim IDEs for viruses that are thought to be in the wild and/or have attracted media interest. Sophos strongly recommends that you perform the monthly update to protect your system against all viruses known to Sophos at the time of publication.
• When a new type of virus is discovered, Sophos occasionally needs to modify the virus engine in order to detect this new type on your system. If you do not perform regular software updates, your system may be vulnerable to new virus types.
• Occasionally IDE structures are changed in line with virus engine modification. Old IDEs could be incompatible with the latest version of Sophos Anti-Virus.