A giant iPad, although having some appeal for the more full sized screen, becomes a mess when you think about real world use. It's so big, you would need to hold it with 2 hands, which then makes it impossible to interact with it.
Or it sits flat on a desk, which is another not perfect scenario for using any device.
As someone who uses an iPhone 6+, iPad Air, MacBook Air (2011 13"), a MacBook Pro retina (2013 15") and a 21" iMac, I find they all fit a number of use-cases. I like the 15" retina not just for the retina screen, but for the usable size (as an almost 50 year old my eyes are not what they used to be). I love the MBA for its portability and it has to be one of the best looking laptops ever made (not that looks matter, but it beats working on a nasty plastic brick). The iMac is still the go-to machine for doing engineering and design work (where screen size makes a difference) and the power and silent operation are also big pluses for me.
Since getting the 6+, I do find my use of the iPad Air reduced, but it still is perfect in meetings, when I want to show photos to several people sitting around a table, something that I at one time accomplished by passing around the mba.
I'm quite sure that if Apple comes out with a 12" iPad with a great detachable keyboard, I could dump the mba and iPad and have a great travel laptop / iPad. That makes complete sense to me.