I can see the case and in fact this is what I did while I lived in the UK. I never bothered with getting a BT landline, and instead used a Vodafone pre-paid mobile for about 18 months. This was great as a call anywhere was around 5pence a minute (which from memory was the same as a peak call from a BT phone). The only issue I had was I had no access to the Internet from home, but I did not need internet access as I had access from work.
The issue that I see in New Zealand is that local calls are free. In my experience, people in New Zealand have an aversion to calling someone on a mobile due to the high cost (it is still 72cents per minute from a home phone to a mobile). So the cost of a land line to a mobile either has to come down, or we all need to get 200 minutes of free time to use for this purpose.
I remember a lecturer say to our class about 8-9 years ago while at University that the telecomunications industry was changing. What was once wired is becoming wireless, and what was once wireless is becomeing wired. Phones are now wireless and televisions are now connected by cable (or at least I guess a cable to a satelite as is the case in NZ)
Just my 2 cents worth.