Maybe that's something to do with the way the setting was tied into the rules, and the fact that the setting period changed (with quite drastic effects) with each edition prior to Mongoose. Coming from a Gloranthan upbringing*, I've always hated
being gregged by setting changes way more than rule changes. It's not so much that "your" setting becomes invalid, after all you chose what makes it into your game universe and what doesn't. It's just that it makes it harder to fit in published products which follow the new official line, so you end up having to be more and more self-reliant as a referee.
*Vinga: There is no need for that. Really.
Your gonna find that with any meta-plot driven game. Shadowrun, WoD, the afore-mentioned Runequest, most published D&D settings; they all advance the basic setting premise
and complicate the integration of later product. Others establish a baseline and avoid changing the timeline while they explore other parts of the world with new supplements; it's your sandbox.
Personally, I prefer a dearth of rules. I like Cyberpunk(not2020) for that. I gives a basic mechanic, fairly generic equipment and all kinds of customization suggestions. I like building on a raw framework, mainly because it's a buffer from "But page 232, paragraph 3 says the Yamaguchi security only have a gun skill of 5 and light SMGs."
But it's like someone else already said, it's mainly just a matter of taste.