Not too long ago, my group had a nearly year-long Rolemaster (FRP/SS hybrid) campaign. We started with likely about 75% of the rules in place, finding Rolemaster’s maneuvering rules too much for us.
After a while, we streamlined skill selection, and simplified how armor worked, along with combat modifiers. We also combined/tweaked some skills.
On and on this went. We added a bit, but we seemed to drop 2 items for every item we added.
In the end, we had a very simple, streamlined open-ended version of Rolemaster that still included our favorite parts (namely, the awesome magic lists and critical hit charts).
When we played Palladium Fantasy, we streamlined. When we ran Rifts, we streamlined. When I ran Pathfinder with my online group, we streamlined.
Now, you might say, “hey, dummy, start with a rules-lighter game, and you won’t have to streamline so much”. But I’ve always had a much easier time effectively reducing and breaking down a rules-heavy game to get what I want than building up a rules-light game. That’s not to say it can’t be done—I mean, I’ve added a few house rules to augment Castles & Crusades, but then again, you can argue C&C is a stripped-down d20 variant in the first place. From more to less. It’s almost as if sometimes you’re trying to see the figure in the marble block of rules—you chip here and there, until the form you want comes out, vs. taking the existing marble statue and gluing your own bits on it
But all in all, it’s easier for me to take something that is rules-heavy or rules-medium and make it rules-light than to take a light framework and make it more complex (and still have it fit nicely). How much more preferable, I suppose, to find the game you want out of the box--but as a chronic and unrepentant tinkerer, that doesn't seem to be an option that often comes up.