I went to school with MANY engineers... In the physics department we shared quite a few classes with them. I'd trust the majority of them with designing things, because they are pretty good at it. Morons are bound to get their degree and build crappy stuff, but they'll probably get put on less important projects, like... Let's say... Trunk struts.
However, it's the physicists, chemists, and lab engineers who determine oil life recommendations based on mean baseline conditions in controlled laboratories, some even more qualified than Nissan master mechanics, or even "Old Joe," (the guy Grandpa used to take his Buicks to, back when he drove Buicks).
But, really, these numbers don't come out of thin air. They're based on research and design, testing, duplication, statistical analysis, thermodynamics, and I'd be willing to bet there's an effin inverse square law in there somewhere (there always is).
If you're changing your oil every 3-5k, then you're indeed simply throwing away money. But if it gives you piece of mind, then, meh, who is anyone to tell you what to spend your hard earned dollars on? From an environmental standpoint, it's probably bad. But China is way worse, so, "no worries mate!"
Here's a decent journal, worth a look, if you're at all interested in the science behind your hobby... Gives insight into testing procedures of motor oil.
Here's a more layman document explaining some of the ideas y'all are hollering about.