I own an iPad 2, iPhone 4, and a Macbook Pro. It's true that there's a lot of duplication of functions and features, but the form factors of each device suit very different uses. Each device has a very particular set of strengths and weaknesses, and although there might be some redundancy between the devices, they really do compliment each other well.
I work full-time and attend MBA classes in the evening. The synergy between these three devices really help me get through the day.
Anyway, here's my take on each device
iPhone
The iPhone is primarily a mobile communication device. It's small enough to keep in my pocket and can reasonably be kept on hand 24/7. It's an absolute essential device for personal, work, and school phone calls, e-mails, and messages. It's also my primary calendar device and scheduling assistant.
The iPhone is also the only device that makes sense as a mobile music player. While I often hook my macbook pro up to the home theater at home, when I want to play music while working out or in the car, the iPhone does the job.
The iPhone has a great deal of application support and a good browser, however, the small screen is less than ideal for any serious reading or web browsing. It would not be practical to read a textbook or do any heavy web browsing on the iPhone. That being said, in those situations where I do not have my iPad and/or Macbook on hand, being able to access the web, a document, or an App that I need on the fly is an absolute god-send. Beyond that, my jailbroken iPhone serves as a mobile hot-spot for my iPad and Macbook!
iPad
The iPad is honestly my main go-to device these days. It isn't as portable as my iPhone, but its portable enough that I can keep it nearby pretty often. I usually keep it locked in my glove compartment when I'm out and about. While I wouldn't type a paper, work on an spreadsheet, or anything like that on the iPad, all of my textbooks, notes, school and work documents, articles etc etc are stored on it, and because of that, I can read/study just about anywhere. I took my car in for service earlier today, popped in some headphones and got some reading done while I waited.
The battery life is good enough that you could spend several hours on it at multiple times in the day and have 50% battery by the end of the day (my MBP would be dead in about 4 hours).
The biggest draw of the iPad, IMO, is the form factor. You can comfortably read and browse on it just about anywhere, and the screen is big enough that you can do some heavy reading/browsing, which is rather impractical on the iPhone. Reading for extended periods of time on both an iPhone and Laptop is very awkward and uncomfortable.
Web browsing and viewing documents and books feels like a novelty on the iPhone. On the iPad, web pages and documents are displayed as intended.
Oh and virtualizing my Macbook, work computer, and home PC from my iPad is pretty useful to, and has been a great help to me a few times.
Macbook Pro
This is my mobile workstation. Its portable, but very cumbersome to bring around and use. But its absolutely necessary for heavy word/excel work, photo editing, and hardcore gaming. The iPhone and iPad are very capable devices and can perform a lot of the same functions as the Macbook, but there are times when a large screen, trackpad/mouse, a full keyboard, and a desktop OS are absolutely necessary. I would not dare touch a document from work on the iPad and risk screwing up the formatting.
There are a lot of compromises involved in bringing desktop functions to a mobile device, and there are times when the iPhone and iPad just don't cut it. My macbook pro also serves as a home theater PC from time to time, a task that neither the iPad or iPhone could hope to take on.
The macbook pro is also the 'home base' of my iPad and iPhone. Most of the documents loaded to my iPad and iPhone are first downloaded to and (if need be) processed on my Macbook.
That being said, the size and form factor of my macbook does not make it a practical replacement for textbooks and documents. Additionally, in times when I need a textbook or document to work on something on my macbook, its MUCH easier to access that document outside of my Macbook. I need all the real estate on my macbook to work on, and switching back and forth between a textbook, document, or webpage and the word document or excel sheet I'm working on really slows things down. When I'm into any serious work, its very helpful to be able to access documents on a separate screen via my iPad. The alternative to doing so would be printed copies of documents.