Well ye I mean everyone is different, but the 13" is the perfect compromise for me. I do a lot of on location work (photographer) so the 13" is perfect as the screen is big enough to ensure sharpness, it is also very useful for tethering in the studio. It is also a great companion as it is very quick and useful for editing although the screen is a little small but useable. Then I usually combine my lightroom libraries together when I get back to my Mac Pro and use a dual screen set up with 2x 23" cinema displays (The old ones) to edit properly with a before and after preview.
Also use it to show clients work and do a lot of graphic design work on it when im away from my iMac at work so its a proper little work horse.
Even Apple in their advertising advertise pros using the 15" and always have done. The 17" doesn't offer anymore that isn't available in the 15" either so if you wanted a more power for say 3d rendering CAD etc it still doesn't offer any more. Just screen res, but like said with the new resolution there is no need for the 2". But I understand why people want a 17" that was my first machine, and I loved it. Had it with a 23" cinema display but walking to uni with it everyday was a back killer like 4 miles. It was the old style one not a unibody one of the first core 2s. Also with the 17" you cant use regular bags to chuck it in so you had to buy a specific 17" laptop bag and felt a rite wall-e walking around with it. The 13" slips into any standard size bag also reduced the risk of being stolen as laptop bags look like laptop bags. The on location stuff was just a pain too as you dont want much kit with you, studio lights full camera kit, tripods etc last thing I wanted was a large laptop to carry around too. So chopped it in (well gave it to mum) and got a 13".
I would love to swap mine in for a 11" MBA because it offers everything I want as a location machine and its very quick, although the disc space is just too small! Once I have all my software installed thats 50gbs so that takes up half the capacity! My shoots are probably between 1-16gbs depending on what im doing, some weddings up to 25gbs so would be pushing it without an external drive and that IMO defies the point of having a location machine. Also the 3rd party SSD alternatives are so expensive it just doesnt warrant itself. Would constantly be swapping stuff on and off it. So I think the 13" is the best compromise, with mine I have modded it so its running a 128gb SSD and a 750GB drive for storage with 8gbs of ram so works very snappy! Its only a 2.4 too one of the first unibodies. I do also use mine as a personal machine so having the storage is nice, when away with work its nice to have content to keep myself happy while waiting for shoots or on the train etc.
Also Apple have got everyone by the balls now. The new ones have everything integrated and are not user upgradable and got iFixit's lowest repairability score ever with having most of the general upgradable components soldiered straight to the main board. Back in the day you were a bit stupid to max out a mac laptop because the parts are so much more expensive than 3rd parties, like drives and ram. But now you have to pay the apple tax on them, as they are non user upgradable which is a shame. Also the processor upgrades in general terms are expensive unless you are using them for high end graphics or rendering. The base models usually offer the best value-performance per pound, but not now you cant upgrade them you kind of have to max them out for future proofing...
