Nah, you don't get it.
The main point of Google Voice is that you give out one phone number and it routes to your various telephones (home, work, cell, softphone on your computer, etc.). When someone calls my Google Voice number while I'm at my desk, I pick up my work phone. If you had a landline at home, you'd pick that up. The caller doesn't know which device you've picked up; that's entirely your convenience. The only time you might burn cell minutes is when you pick up your mobile, but I think some people add their Google Voice number as one of their calling circle/favorites.
Also, you can pick up the call on your computer (headset equipped) if you install the Google Chat client.
However, I've noticed that I occasionally am not charged if I pick up a Google Voice call on a phone equipped with a Truphone SIM. I don't know why this is the case, but sometimes I am getting free calls. Also, you can forward to a Gizmo5 number. With the proper SIP client on my iPod touch, I can call back to the United States from abroad for free.
Google Voice has other neat features such as switching phones, customizing answering behavior, good antispam protection, voicemail transcription (which can be hilarious), conferencing, and more.