Here is a post from Macworld in which the author argues and explains why his MacBook Air is still his go-to device while the iPad, regardless of whether it is the mini, Air, or the yet to be released Pro, still cannot displace his Mac still. I have read enough similar posts to see an underlying theme: keyboard.
That's it. The Macs have keyboards where as the iPad inherently do not and needs a clip-on case keyboard or one hooked in via Bluetooth. And here is the thing that I have thought about that makes the MacBooks a better option than the iPad with a keyboard. You cannot put the MacBook on your laps as easily as you can with an iPad with a keyboard. Really, think about it. It makes sense.
Having said that, at the end of the day for a lot of these posts, it's about keyboard and navigation versus no keyboard and navigating with fingers and touch.
But Macs are generally more powerful and capable as a computer and, therefore, are more capable of supporting more tools. Take Xcode for instance. If a developer wants to write an app for the iOS ecosystem, his or her only option is the Mac. To me that's a good reason why one would stick with a Mac. Until Apple is inclined to release a iOS Xcode app, we really won't know what developers will choose, keyboard or not.
Oversimplifying this a bit? Maybe. However, this issue could be generational. Those born into the touch UI might well be fine with typing on a glass screen or feel quite at home dictating to Siri. Those of us older who started their computing experiences on a keyboard would like see things differently.
It all comes down to preference. I'm on the camp that uses my iOS devices fora cats majority of my computing needs and into go to my Mac when I am absolutely have to.
Note: This post was writing on my iPhone.