You can't take your "landline" with you! That's all the reason I need to have both.
Seriously, I got my first cell phone while I was away at college. My mom felt better with me having one in the car driving back and forth to school during my trips home. It was "in case of an emergency" that I got it.
Now, I use it mostly to make and receive long distance calls, and sometimes I still have minutes left over. I still have my number from my hometown so when my mom calls on my cell number, it's local for her even though I'm three hours away. She just loves it because she doesn't have to pay for it on her end.