Buying an unlocked iphone is useless if i currently dont have any data plan?

Not really. You can mess around on it with wifi. I did that for about 5 days till I got a plan.

Can still use every other non data required app or function of the phone. Calls and texts wherever you have service. It will be like a fully functional iPhone whenever you're on a wifi area.

But you can't use a smartphone without a data plan on the major carriers.

it'll just be an expensive iPod Touch until you put it on a carrier

Some carriers, like Verizon, automatically add a data package when it sees an iPhone is added to the plan. Call your carrier to ask.