Yeah, but that sucks. You can't use the native GV feature when calling out. I think it's a great idea to use Sprint to home if you really had to penny pinch on minutes, but for me that would be too tedious.
GV is awesome as it is. My GV number is my main number, I no longer distribute my actual phone number (except to receive MMS, but I only get those from the wifey). For me the real value is when I'm working and I receive text messages or I get called... I can seamlessly move back and forth between the phone, and my work console/work phone. The other party doesn't know which phone I'm using--what's REALLY cool is the ability to move around from office to office, and whatever landline is available becomes my newest GV phone.
It's really an amazing work-productivity tool. If you don't use it for that... it's just a cool "phone-in-the-cloud" service.