Both my iPads are cellular ... It depends on your situation, I live in a rural area with full strength 3G from a new nearby tower, before that had a terrible satellite system. So if you live in a city where you can have fast wifi, you might not need cellular iPad, especially if you have a smartphone. The other reason for cellular on iPad is if you use it for GPS, as I do.
I love the fact that my iPads are always connected, I live in double storey house, so have one iPad upstairs, one downstairs, have access to fast wifi in town for software updates etc. I tether my Air from iPhone or ipad4 when I need Internet on that at home. SIM cards on yearly plan in iPads, really cheap, and monthly plan for phone, also really cheap. In total, less than $45 a month, compared with $120 a month before for landline, mobile phone, satellite wifi, and iPad 3G. This is in Australia, so might be different for you. And my setup obviously isn't for streaming movies etc.