Right, linking more 10-digit numbers will cause Google Voice to simultaneously ring all those numbers, in addition to the web client, the app clients, and the OBiTALK clients.
Conditional call forwarding is necessary so that, when a call is sent to a linked number, and that number is busy or unanswered, the call will then be forwarded back to your Google Voice voicemail. Otherwise, if you are on a call on that number, its own voicemail service will take the message.