You have just finished a solid draft of an IVR call routing application, and you're pretty happy with. The menus follow conventional wisdom for numbers and types of options, and the design has done well in early usability testing. You're looking forward to sign-off on the design, and one of the business analysts pipes up, "Hey, we need to add one more option to the main menu. What's one more option?"
"What's one more of anything," like records in a database or fields on a form, usually isn't a big deal. Computers are supposed to be able to iterate any number of things with a minimum of additional effort, so the same principle should apply to IVR menu options. That's the logic, anyway. Unfortunately, the same logic applied to IVR menus is a disaster, because it's the callers who have trouble sorting through and remembering and verbally reproducing multiple menu options.
The logic of "What's one more option" is very seductive, and can lead to main menus with ten or more options. Big main menus (or "Maim menus," for what they do to callers) just scream "I don't know what I'm doing! Hit the zero key now!"
IVR menus structures are still actively researched. For an excellent recent study on IVR menu options, see "A comparison of broad versus deep auditory menu structures," Commarford et al., (2008), Human Factors, Vol. 50, pp. 77-89.