What you're missing is that you now only have to set up one account to support call waiting and three-way calling. That means the other SP can be used with a different Google Voice account (maybe you'd like another GV number in another location?) or with another VoIP service (commercial VoIP provider, extension off SIP PBX, etc.).
And for those that happen to be using the device as a gateway between Asterisk (or some other SIP PBX) and Google Voice, you now have two channels per SP instead of just one.
And even if none of that interests you, it means that call waiting and three-way calling with Google Voice just works, without the need to configure identical information on SP2.