I find this very hard to be believe. In your bag? With books, notebooks and whatever else you carry around with you? I certainly don't notice the difference between the 13" and 15" in my backpack or satchel. If you're backpacking or travelling, I suppose, but normal "life"? Seriously?
Also, I think you're underestimating how much space those 2" actually give.
Depends on how you define normal life. I don't do backpacking, but I frequently have to walk (except I could always use other methods of transportation, but I'd prefer to walk whenever possible) for an hour or more in the city, and weight differences between laptops can be easily felt. The weight difference in between 13" rMBP and 12" rMB is just 660 grams as well, and I can easily feel that my load is lighter.
I admit that if you use a backpack, then the weight is distributed more evenly, and the difference becomes harder to feel, but it still can be felt. I often use messenger bags, in which the weight difference is obvious. Also, I don't carry books that often, but I can see the point that a heavy load of books may make it more difficult to discern weight differences in between different laptops. However, in such a case I'd especially recommend to go for an as light laptop as one possibly can go, just to lessen the burden at least a little bit.
I understand that 2" increase in screen size actually gives appreciable amount of extra screen space, and now in my case a 3" increase would be even a greater difference as rMB has just 12" screen. Yet, at least in my workflow, there is nothing that I could justify spending $700 more to gain 3", especially when the 15" rMBP would be 1.12kg heavier, and also so big that it couldn't even fit in any of my bags. I also admit that I value mobility very highly.
Yes, 15" rMBP is definitely a better machine for work all around, but it is expensive and heavy. If money is not a concern, it would indeed pretty much always make sense to go with the 15" rMBP as long as you don't need to carry it around too much. However, you outlined that if one is going to consume a lot of media on the laptop, then the 15" rMBP has a clear advantage over 13" rMBP. Again, yes, 15" rMBP is going to be better for this purpose than 13" rMBP, but I would argue that you have to value that media being consumed a lot in order to justify almost half a kilogram increase in weight, and $500 price difference in between 256GB 13" and 15" rMBPs, or $700 difference in between 512GB models. In my opinion, these price differences will be better spent elsewhere, but people have different needs and I can see scenarios, where 15" model is really beneficial and easily justified, media consumption just isn't one of them.