Well, you're right that the iPad doesn't do anything new that your MBP or iPod touch can't. But then, a Windows machine can do everything an OS X machine can, so why do people use Macs? A compact car can't do anything new that you couldn't do with your minivan or SUV, so why do people buy those?
My iPad replaced my netbook -- an Asus EEE PC that got 10 hours of battery life. I bought a netbook because I wanted to be able to access my email, surf the web, get to my files, do slide shows and presentations, watch videos, etc. while on the road -- either on weekend road trips, or just when visiting friends and family. I could bring my MBP, yes, but I always worried that my life would be in ruins of all of my data on the MBP got lost or damaged or stolen. Actually, the scenario that bothered me most was leaving the MBP in the car all day, during a cold Canadian winter, because I would need it later that evening. So the netbook was a $400 PC that I basically didn't really care if it got lost or damaged, and it was lighter and easier to carry.
But slow. And clunky. And virus prone. (Yes, despite my best efforts, and despite never surfing anywhere "questionable", I did manage to get infected by a virus. Twice.) Don't get me wrong, it was workable, and life was good.
Then I saw the iPad. It was half the weight of my netbook. A third of the size. Equal runtime. Equal screen size. Roughly equal capabilities. (Sure, the iPad doesn't do flash, and the EEE PC did -- but only barely. It was slow and bloated. And sure, the iPad had no keyboard, and the EEE PC did -- but the netbook keyboard was so cramped and difficult to type on that I called that a draw.) And most attractively to me, it offers me a way to get 3G data on a device for only $15/month, and with no contract. (Note: Canadian carriers.)
I played with one in-store, and was skeptical at first, but the more I tried it out, the more I could see it completely replacing the netbook. Which is exactly what it has done. I sold the netbook last weekend.
I'm typing this message at work. Where I work, access to personal email is blocked due to security issues. So is access to Facebook and the like. With the iPad, and my 3G access, I can now check those during the day. The iPad tucks away discreetly in the corner of my desk, so the IT guys don't get mad at me for having an unauthorized laptop.
So the iPad follows me around now. I check my email or do one last Facebook update from my bed, just before I go to sleep. It's here at work, ready to let me check my personal email. It'll come with me when I visit my family tonight. On Sundays, it comes with me to church and I read a Bible on it. It'll go with me to the cottage this weekend, and I won't even take a charger -- the battery will carry me through.
Sure, you can do all that with an iPhone, but I wasn't willing to sign onto a 3-year (Canadian) contract at $65/month (almost TRIPLE my existing cell phone bill for a no-contract, no-frills voice plan). I can see that if someone has an iPhone, the iPad's not that big a deal.
So it's not a device for everybody, but it's not useless for everybody either. If you've already got a MBP and you're happy to carry it everywhere, and/or if you have an iPod touch or iPhone, then sure, the iPad is less useful to you. But if you don't have one or the other or both -- or aren't willing to take them around everywhere -- (e.g. my circumstances), then the iPad is a great "in between" device.
Then there's the sheer fun of using Google Earth, Google Maps, even Street View, just by rotating and zooming things with your fingers. Google Maps are also much easier to see on a 10-inch screen than a 3-inch one. This is where I argue with the "you can do it all on the iPhone too" people. Sure, you can tick off "access Google Maps" on the checklist for both the iPhone and the iPad. But have you compared the experience on the two screen sizes? Google Maps is so much nicer on the big screen.
So in your case, if you're happy with what you got, then no, you don't need an iPad. But don't call it "useless". At least, not until you've tried one for yourself.
