Then what about the air and new MacBook? Surely if the rmbp 13 is only for everyday tasks then it's redundant.
You are asking the wrong people in the wrong place. This is a site filled with tech geeks that think that you need the best of everything to do things the fastest they can be done. Of course if this is your business then this is true, the time savings are worth the investment if your livelihood relies on your computer productivity.
However if you are just a business user using office, doing presentations, remotely accessing your companies servers etc the 13 inch is perfect. Likewise if you are a developer travelling around or a photographer / blogger etc doing editing on the fly. An engineer that needs a slew of apps for the machines they are servicing, would be very happy with a 13 inch.
And most home users who want something great for all their needs are very well served by the 13 inch, edit those go pro videos, run almost any app available, basic gaming etc they are brilliant little laptops.
However if you are doing 3D CAD rendering for your job then the 15 inch is the way to go. Not that the 13 inch can't do the job, and do it well, a quick look at the AUTODESKwebsite shows that just about any computer made since 2010 will run it, it's just how fast it will do it and how much it will bog down you computer running it, when you need to do other things.
As for the rMB and the air they are very much like the 13 inch rMBP but only able to perform as such in smaller bursts before they throttle back. The Air is redundant (although still a remarkably capable little laptop) and the rMB will supercede it as the processing power in the core M line grows.
As with all computer purchases you need to know your software well enough and your workload well enough in order to buy the computer appropriate for your needs. If you don't know this information then google is your friend and a couple of hours invested in learning this information is time well spent in the furtherance of your carreer.