I'm sure there are many reasons, but for us when we are downstairs in our house we do not get much of a signal on our phones, but I can make a call thru our wifi with no trouble. So I guess if you were in an area where there was no service you could go to a hotspot and still use your phone. Good question.
Mainly because WiFi is faster then 3G. You may also be on the road in a place without 3G but you can walk into a Starbucks and use the WiFi network. No matter where you are you will find dead spots. If you live in one of them you can use the WiFi.
Unlimited cellular data plans aren't actually unlimited. If you look at all the fine print you will find a cap (5GB if you use Sprint). That data cap is for 3G only and doesn't include the data obtained via a WiFi connection.
It depends on the phone and how you have it set up. I believe that is an option. A word or warning though, turning on the additional radio will lower your battery life. I always recommend turning off Bluetooth and never ever ever using it for any reason including hands free headsets. If your is on and you turn off the Bluetooth radio and turn on the WiFi radio it should pretty much wach.
hahaha! yes sometimes we are getting disappointed because of the ability of our phone that we wont expect, a kind of ability that can give you a lot of work that you should not expect too.