One reason is that the same code base using the VC2005 compiler will resullt in most cases in better code generation (higher performance) since you can just trigger the newer optimizations.

Also, moving your code to newer supported version now makes it easier for you in the future to make use of newer technologies. E.g. Extending your MFC apps to use Win forms as well.

This is only part of the benefits you get from moving to VC2005.

Thanks,
Ayman Shoukry
Program Manager
VC++ Team