Apple - What is assistantd?
According to this site, Ongoing Cross-Section of assistantd Headers - Useful for Siri Mods, it appears
assistantd is associated with Siri functionality on iOS.
Given this, I suspect
assistantd on Mac OS X is related to the new dictation functionality. Dictation on Mac OS X 10.8 uses Contacts and other personal information to improve accuracy; gaining access to this information would explain the need for Keychain access.