I disagree. iPhone + Laptop/iMac can do everything better than iPad, so why would you need an iPad for a "third wheel"?
It needs to have something special that the iPhone doesn´t have (bigger screen isn´t that special feature), otherwise it´s just a very stripped down version of the iPhone and you´re just making excuses for it.
Things the iPad does better then iPhone and laptops.
Web surfing. Much more enjoyable. iPhone too small. Laptop more cumberson. iPad hands on and in my lap.
Watching video. Air video is the most awesome thing ever. I have two terabytes of video available on the iPad where ever I am. iPhone too small. Laptop too hot in bed, start fire and kills your sperm. Ipad awesome for watching video almost anywhere.
Office use on the go. If you are actually having to do work while on the go then the iPad beats a laptop and iPhone hands down. Example, having to collect and input data into a spreadsheet while in the car (I do this). Laptop is to difficult to manage in a car. It either has to sit on the passenger seat and you twist your body or you have to push your seat back to put it on your lap, and then pack it away so that if it falls it doesn't snap the screen off. iPhone too small. iPad allows you to hold it in your hands and type and then just either throw it next to you or leave it on your lap.
Email on the go... Same as office use on the go. Laptops are too much work due to their form factor. That is why so few people actually use them as portable devices. Most people use them as desktop devices which they move around. iPhone, or any phone sucks for typing because you are just thumbing it out. I am typing this on the ipad with all fingers like a normal keyboard.
Guess what, none of these needed a camera, front or back, or a gyroscope to work. They didn't even need more ram. Retina is irrelevant since we already have more pixels then the ip4.
Bigger, is actually better.
All of this boils down to the fact that you cannot afford an iPad, so you are trying to justify your not having it.
If you like the iPhone, the you will like the iPad more. That is unless all you do with the iPhone is take photos and play just released games with 6axis rotation. If that is the case, I suggest you sell your iPhone and by an SLR and PS3 since they absolutely kill the iPhone at those.