I still use my iphone when I am on the move, because it is easier to use and keep/whip out with one hand, and a lot less bulky compared to the ipad.
The ipad is more when I have the opportunity to sit down, so the extra bulk isn't so much of an issue. For example, if I am sitting down marking my books, I may prop up my ipad and run shadow era (an online CCG).
I can't imagine any situation besides surfing in a chair that the iPad does better than any of these devices?
If you surf a fair bit, that reason alone makes all the difference. For me, the ipad is my backup when my dad needs to use my imac for his work.
There are also some cool combos you can use together. For example, if you have airdisplay, you can use the ipad as a 2nd display for your macbook.
In short, to quote Steve, the macbook already does most of the things your ipad will be capable of doing anyways, just not as well. I won't say it is a waste of $700, but again, it ultimately depends on how much you really need that money.
I have friends who went with the entire mobile package - 11" MBA, 15" mbp, iphone and ipad, and he uses them for separate purposes. MBP for work at home, MBA for when he is on the run outside, iphone as a phone, ipad for relaxation at home, and icloud to sync everything.
