Samsung, like most other phone makers, seems afflicted by the notion that people want them to shit up their devices with ugly UI, custom skins, and ridiuclously unnecessary applications and features. Nobody can figure out why, but it seems to center on some concept of "differentiation". Sort of how I can "differentiate" myself by farting on an elevator. As a result, people end up wanting/needing to root and flash their phones with a bunch of user driven ROMs in order to get closer or match the intended google experience. This is generally ok, but the people who work on the ROMs end up needing a fair amount of support from the device maker to make kernel source available so that they can actually create these roms, and the companies haven't been great about releasing source in anything like a timely manner, and this all gets even more complicated when you consider that the carriers also feel a need to "differentiate" and add on more crapware which only further complicates the kernel source situation further.
Therefore, the buzz and promise of getting a pure google S4 would be the idea that you get out of this shit cycle and just get a great phone with the latest version of the software. With timely updates. You're absolutely correct that if google is doing yearly updates, then you don't end up outdated all that long, but you're probably looking at it wrong.
Take the phone I just picked up, the HTC One. HTC released it on android version 4.1.2, which is already not the latest version, because HTC insists on making their skin (Sense, which happens to be much better that Touchwiz, but I digress) and their own software customizations, which were probably built against android 4.1.2 when they started, but 4.2 was released in the interim. Now today, google seems to be getting ready to release 4.3, and it's pretty likely that HTC will probably be already in process of getting some 4.2 version of the update ready, which they'll release sometime between now and mid summer. Then they'll be started on 4.3, but you'll likely not see that until winter at the earliest, just due to all the testing cycles. And this is on a major, flagship device, if you own something a bit less sophisticated you'll be lucky to ever get some of these things.
Which only becomes a problem when google innovates. Android 4.2 (finally) introduced bluetooth low power spec, but phones on 4.1 (like the brand new HTC One) can't leverage it, making a bunch of potential accessories not work, car stereo integrations worse, etc etc. There's a host of new features being announced by google today that won't be on major android devices until next year, when they're already preparing a bunch of new innovations. For most people, this doesn't matter, but for people who give a shit this is of course very frustrating.
Therefore, this long winded response is a great way of saying, yes, you're right, this isn't a monumental deal for most people who aren't going to want to use the latest and greatest, but since the SGS4 and HTC One and these flagship devices are trying to cater to the mass public AND the enthusiast, the enthusiast types who want the latest and greatest are going to go nuts because they can get the SGS4 without all the ROM/root/slow updates headaches. In essence, they can get a "Nexus" S4. Finally, this is a bigger issue because the actual existing Nexus, while an awesome device, isn't LTE enabled and ahs some of the other limitations that people who love android consider non starters, so this essentially makes a new nexus device without a big change to the nexus program.
I don't know why this required an essay and you're one of the smarter people here so you already know 90% of what I typed, but I guess I figured I'd lay it all out there.