There is always going to be a newer/faster/better device just around the corner...
For me and my uses, WiFi makes more sense than 3G/4G. I already pay for unlimited data on my phone. I don't need to pay for unlimited data on a tablet at the same time, and the amount of data I'm using on the Xoom would probably mean going with an unlimited or at least high volume plan, whereas WiFi is free (and faster) as long as it is available.