That depends on situation, My phone keeps me accessible, something that would never happen if I were only reachable by landline, it helps me coordinate and re-coordinate plans instantly and has helped in more than one emergency.
Do I have to have it? No, but it saves me hours of hassle, travel and stress daily.
Plus with the all new features they add every day I'm just waiting for the built in pocket knife feature. Then I'll really be able to depend on my phone.