Interesting analysis. Would you say it somewhat depends on *when* you sell? I can see what you're saying being the case when it's 3-5 years down the road, but in 18-24 months you still think it isn't a benefit?
Also, would you say that it at least keeps ~$200 in value or at least close to that?
Good question! I would say that timing matters, but it's more important with respect to the announcement/release of new models than an absolute number of months. Specifically (and intuitively), unloading prior to a release will net you quite a bit more than waiting until after it. (Tangent warning - if you don't care about some anecdotal numbers, skip to the next paragraph.) I used to keep a spreadsheet of my sales prices versus my purchase prices (including RAM and accessories, and after factoring in discounts). When I sold before releases, I made $160 on one and was dead breakeven on another. When I sold after releases, my upgrade costs were $537, $292, and $397. My latest upgrade (from February 2011 model to Retina) will cost me $375. I bought the base 2.0Ghz model last time and put in an aftermarket SSD. The fact that it had 8GB of RAM versus the standard 4GB netted me very little in terms of my resale value. /endtangent
But to answer the question as you put it regarding months, I would say that you'll get part of the $200 value (maybe as high as $150 -- closer to the $200 during these first few months) if you sell before the release of the next revision and during the next 12 months (i.e., standard warranty), and I would guess you can command $50-$100 of it after the next revision, which aligns with your 18-24 month timetable. The problem is that there's a psychological barrier people have for paying close to or above market values for "new and improved" for "old and used," even if the old and used has a feature that's better than the new and improved. (This doesn't hold in all cases, and I can come up with lots of counter-examples -- but on average, it's true).
I would say the resale question makes the most sense put into a specific context. Assuming there are no price drops on the (probably) Haswell revision next year, your extra 8GB of RAM aren't going to seem that important in the wake of Apple's and Intel's marketing about their groundbreaking new chipset that's "XX% faster than the previous rMBP". Anyone who cares super deeply about performance is going to buy new, whereas the used market is simply more interested in bang for the buck. It's the same reason the people getting the souped up $2799 models won't get anywhere close to the extra $600 they're paying now on resale.