I have an iPhone, iPad, and MacBook Pro.
The MacBook Pro is for computationally intensive research tasks. Programming, etc. And because I don't really have furniture, I use it for general browsing when I'm home, too, since it's usually at least as close as my iPad, and it's still better for multitasking than an iOS device.
The iPad is for reading journal articles and books, taking notes (it's hard to write mathematical equations quickly on a MBP or iPhone), and I generally leave my MBP in the office or at home and carry my iPad and iPhone when on-the-go, when I don't need the computational power of the MBP, so the iPad is my portable computer when I don't want to carry the weight of a 15" MBP and need something I can whip out quickly. It's also great for showing images and slides to illustrate results during small meetings.
And the iPhone is (naturally) for phone calls and messaging, for setting up meetings and reminders (with Siri now!) when I'm not at the desk, and for checking things quickly that don't require the iPad's larger screen size.
Since I need to be portable right now, they all make sense. If I had a desktop instead of a MBP, then it would be a much harder decision to choose between an iPad and an MBA.