For me, the biggest reason I still take my MacBook with me when I need to get work done on the road is simply: typing. Whenever I expect to spend quality time with a keyboard, I want something beyond the iPad's virtual layout. Although Bluetooth iPad keyboards, such as the Zagg Folio or Logitech's Ultrathin Keyboard Cover go a long way toward addressing my concerns, they are not entirely satisfactory. For one thing, typing is a task that almost never benefits from a touchscreen. Every time I have to use the iPad's loupe tool, which I still do even with a physical keyboard, I mutter "fail." And, although the iPad has excellent text processing apps, they all fall short of the capabilities of the ones I use on the Mac.
One of the biggest limitations of the current version of iOS is that it cannot truly multitask. While some may prefer the simplicity and focus of "full-screen" mode (and Apple certainly promotes it), I don't. I would much prefer to be able to view my Twitter feed at the same time as I am writing an article. And I would like to be able to copy text from one document to another with both files viewable simultaneously. While all of this is easily accomplished on a Mac, it is currently impossible to do on an iPad.
While these are the major limitations, numerous lesser ones may still be critical for some users. You may have essential apps on your Mac that do not exist for the iPad. You may not want your app choices to be limited only to what Apple permits in its App Store.
Still, for people who own both a desktop Mac and a laptop, I believe the day is soon approaching when most of them will give up their MacBooks for iPads. But the day when the iPad can replace every Mac is still not even on the horizon.