I get hung up on a few things in your post, lol..."Get you stuck with Ivy Bridge processors"...you say this as if it's a bad thing! LOL...the Ivy Bridge processors are amazing, hell, even the Sandy Bridge processors were great. Obviously newer gen processors will bring some pretty cool functionality to the table that these two haven't (including substantial battery life increases from what I've heard), but the term "stuck" with either the Ivy or Sandy bridge processors is a bit misleading, as they are really great setups, and will remain relevant for at least a few more YEARS.
It's a fact that the Macbook airs are currently the only laptops from Apple that have been updated to use the latest Intel processors. The OP is certainly free to use whatever brand he so desires; I just felt it was worth pointing it out so he could make a more informed decision. If he opts to stick with Ivy Bridge, it must be from a conscious choice on his part, and not because he is unaware that a better alternative exists.
And then there's the notion that "few people actually" upgrade their machines with RAM and SSD on the non-retina display model of the MBP...what difference does that make? I bet you even fewer people would even pay attention to the processor change that comes in the next round of MacBooks, so if you want to talk about who does what with their computer, I'm guessing those same people who don't upgrade their internals won't care about the processor names either, they simply see "i5" as the base line and "i7" as the top end and make their choice on pricing differences.
That's like saying it's okay to sell fake abalone because they taste the same and people can't tell the difference either.
I don't need to know that my macbook uses haswell for me to benefit from the longer battery life and improved graphics performance. It's there whether you know it or not. As you use it day in and day out, the advantages of Haswell will affect your daily performance and workflow.
The distinction here is that when I make recommendations to people, my advice is more geared towards providing the best experience "right out of the box", with as little hassle as possible. This means that while it is certainly possible to buy a normal MBP and manually upgrade its internals yourself, I will rarely suggest that because I don't know how tech savvy the user is, nor do I feel it is fair to expect him to have to go through the hassle of ordering extra parts from OWC and having to crack open the macbook to install them. Lastly, I cannot be held responsible should something go wrong, so I feel I must be very careful with my words.
The fact is that you're looking at only around a 10% difference between the i5's between haswell and ivy bridge, and mainly for graphics integration...the i7 option on the MacBook Pro's is still a better option for performance even with the Ivy Bridge, and if the i5's have shown us anything, Ivy Bridge processors will remain perfectly sufficient for quite a while after the new Macbook Pro line brings the haswells to their setups. Now that's not to say the haswell computers won't be worth every penny of their cost, just based on battery life improvements alone I'm sure they are, but we're not talking about all single core intels here, these Ivy Bridge Macs are still completely relevant.
Well, is there any benefit of going with a MBP over the latest airs for photography? To me, the airs would represent the best all-rounded combination of weight, size, battery life and specs.
No offense, but there's pretty good evidence that shows people who pay attention to the newest processor specs as closely as you've pointed out are probably ALSO the same people who would do things like a simple SSD swap or RAM upgrade on their machine. We all have opinions, and yours being that the Air is the best out of box experience is fine...I just disagree, and the Air doesn't meet my needs on many levels, especially since it's missing both the ability to easily upgrade the internals AND the option to have a retina display model if that was something that mattered to me.
The retina model is as locked down as the Air in that both have soldered ram ram and lack a cd-drive that can be swapped out for another storage drive. If the OP is going for a retina MBP, then all the more reason to wait for Haswell. Those high-res screens suck battery life like crazy, and we will probably see a substantial improvement to both battery life and graphics performance with Haswell, especially since the 13" version lacks a discrete graphics card.
Likewise, it is not my money, so I generally shy away from recommending the more expensive options unless I feel there is a genuine need for such a high-end product.
I mean my mid 2012 MBP is now running a 256GB SSD and a 1TB HDD for general storage...now I'm sure most people would never need that kind of internal storage on their system, but the kicker is this, I spent less on both of those drives with the dual drive adaptor that takes the place of my super drive than a person would upgrading to JUST the larger SSD on either the Air or Pro's from Apple.
And the catch here is that you had to add in that SSD drive yourself, which mandates additional effort on your part. Unless you are volunteering to be there in person to help the OP install the SSD in his macbook when it arrives, I won't insist on such a route, exactly because I don't know how tech-savvy the OP is.
I am sure we all have his best interests at heart, just that we go about it in different ways.