I'm using a MBA (rev C, SSD) as my main computer, and an iPhone as my secondary computer. I work on my MBA and I browse the web while in bed with my iPhone. The iPad has an advantage over the iPhone, it's screen size, but has many more disadvantages, less portable, no camera, no phone calls, probably worse battery life. I'm always carrying my iPhone around because it is my cell phone, so it's always there when I need it. I wouldn't carry my iPad around unless I knew I would need it beforehand.
The iPad may be a good replacement for the MBA if you have another computer and you are evaluating a MBA as a secondary computer for light tasks. But in that case, the iPhone would be an excellent replacement for the iPad and the MBA. The MBA is a good replacement for a main computer, but I wouldn't buy it as a secondary computer, because of price.
If you have enough money you could buy all of them: MBP, MBA, iPad and iPhone, and use the best one for what you're going to do.