The trouble with a completely systemless ruleset is that you need examples of how the rules work.
Sure, you could have generic and examples of that, but it is a lot easier to model examples on settings that people are familiar with.
Even Traveller had a setting and wasn't generic.
For instance, if you wanted to make a distinction between humans and aliens, a common enough thing in space games, you'd have to make up some alien species (I'm trying to avoid the term "races" because it isn't race, but it's difficult) so you make up a completely generic species and have examples of that with no cultural background or something completely made up. As opposed to made up SciFi species, of course.
It would be better to give examples using familiar settings, where possible.
The problem is that people generally want money for settings, so you'd have licenses and so on, which ties Mongoose in to particular deals.
So, perhaps a systemless setting isn't such a bad idea, even though it galls me to say so.
What would be good, though, is for fan writers to write up sepcies and settings and publish them on websites, making no money from them, as examples of settings. If you pick settings where the author has died or the work isn't being used, then there shouldn't be many problems.