It's not pointless to have 2 different phone services. There may be times (natual or man-made disaster) where one type of service has failed, but another is ok. A fire in your home may destroy the landline wires, so you call the 911 with the cell; weather may knock out a cell tower, but your landline can still work, etc.
My wife and I thought about going cell-only, but it's not that great a choice if there is an emergency and you can't get service (tower getting hammered or it's faulty for example).
If you have multiple phone numbers, you can use Google Voice once it's fully available or you get an invitation. You'll have a third phone number from Google, but that is the only one you need to give out. Depending on how you have it set up, when someone calls the Google number, all your phones can ring (landline POTS/VOIP/etc AND one or more cell phones, etc) and you simply answer from which device you want.
What's really cool is say you answer on your landline, but need to go out and the call is THAT important that you can't hang-up - you can easily transfer the call from the land line to the cell.
I understand where you are coming from, however I had a Vonage line for 3 years. Barely used it. It was costing me 30 a month for it and it was a waste. Service was crap and I got tired of dealing with "support" from india. Could not understand them and when I went to cancel, they said ok, you are canceled. Yet months later, I kept getting bills in the mail and then finally a collection notice. Exactly what I need from a company like that. No thanks. Never again will I deal with them or any other VOIP company.
I don't have a need for a land line. Emergency or not. If there is a natural disaster to the point where mobile phone towers are jamming up, the land lines will also be screaming busy. I don't need another number.
I have had Google Voice/Grandcentral for years now. It is really nice. I have also used the feature of transferring the call to my mobile from a land line before. Nice. However - a mobile only phone works just fine for me.