You aren't covered if your code is revoked and past the one year original warranty and passed the time frame for a dispute. At which point you have left positive feedback and can't change it. And so you know, the problem with the codes are not immediately evident, at least to Apple Support. I do not know where the problems are detected, but I know that it happens months after it is registered. See the threads in the Buying Advice section.
https://forums.macrumors.com/threads/690859/
The positive feedback of these "code" sellers is not in anyway indicative of the long term satisfaction of their buyers. These problems have been happening months after the code was registered. If you are convinced that the feedback of these sellers is an indication of the legitimacy of these codes, your standard for trustworthiness is rather low.
I have no doubt that some make it through the system with their code. But no matter what the percent of successful codes, I don't want to take the chance of being in the percentage of users called out. Because when that happens they demand the serial number from the box, which of course you can't provide. It also leaves you, in most cases, unable to transfer the warranty in the event you sell your mac.
To close, your insinuation that I am somehow foolish for paying "nearly $100 more" for a box copy didn't go unnoticed. If you must know, I have never paid more than $40 over the cheapest "code," and I would much rather pay that than be left with no warranty.
And don't you find the marketing techniques of these sellers strange? They use a picture of 20+ applecare boxes (suggesting they have the boxes), but they only email you the code. And their reasoning for this is to be "green," or "you don't need the box" (which you actually do need). And when you email them to send you one of those 20+ boxes they have in the picture, they don't do it. Usually it is because they throw them out or whatever other excuse they have. Here are some examples from eBay sellers:
Here is costal_computing, a seller who asks you in his listing to send him a message if you need a box. A link to an example:
http://cgi.ebay.com/ws/eBayISAPI.dl...&ssPageName=ADME:X:RTQ:US:1123#ht_1562wt_1167
This is what he replies with when you email him asking for a box copy.
Here is another, this time ameenterprises.