I played most roleplaying games when they were fresh in their first edition: old Runequest, Pendragon, Arms Law and Spell Law (before they became Rolemaster), Dragon Quest, Powers and Perils, Call of Cthulhu, Werewolf, Warhammer, Advanced Dungeons and Dragons, so I have plenty of experience playing with core rules only.
However, in most cases I always prefer using added material if the game is good. The only game I would enjoy playing stripped to its basics more than complete would be original Pendragon. Original Pendragon was just knights errants: no races, no casters, no rogues. That and its faithful recreation of Arthurian legend made it unique and the best roleplaying game in the history of mankind.
But nowadays I prefer 4th edition where everything is core.