for most of the time the answer is : you do not need a land line for your house if you are happy with your cell phone.
you can also get a Voice Over IP telephone line connected to your cable modem that will sound just as good as a real land line but will be much cheaper and will offer much more services (Vonage, lingo, BBTelSys are good examples)
They usually include unlimited calls, caller ID, etc and BBTelsys is $200 for 2 years of service!)
-In case of power outage, the VoIP will not work unless your modem and voice adapter are on a UPS.
-In case of major disaster, the cell phone may not work.
-if you leave someone at your house without a cell phone, and they need to call 911, they won't be able to.
So there are a few cases where a traditional land line is useful. Not very often but when you need it and can't have it the consequence can be bad.
I have a DSL and phone line paid for by my company. All our phones are connected to VoIP lines on the back of the DSL line. I can always use the comapny phone for emergencies.