
Voice and video were the killer features of Yahoo that led me to use it regularly. People will still have contacts in different IM networks and switching is rather difficult. If Google opened up the gateways, then you would have a reason to switch.
I just found out there is an add-in for both Google Talk and Skype that adds video conferencing to your IM experience: http://www.festooninc.com/
I haven't tried it yet but we'll see how good it is.