It seems to be the fate of many RPGs to see the corebook eventually strangled by a sprawling mass of splatbooks and updates/changes to the core rules, wherein new material is just plain better than the old material, and suddenly you show up for game night and somebody is playing something that is like your character, only your character sucks now.
3.5: Screw your fighter, and screw your longsword... I've got a Warforged Warblade with an Elven Thinblade. I just used the words war and blade far more than should ever be used in the same bladewarblade sentence. War. Blades.
Oh, and I do more damage in a round than you do in most combats. Bitch.
RIFTS: You down with OCC? Yeah, you know me. Mystic, meet Grey Seer, who is like you but with better spells, more abilities, and auto-dodge. Suck it.
Welcome to South America. We have Ghost Wasp Armor, gringo. Fuck your SAMAS.
Vampire: Yeah, we're the True Brujah, muthafucka! You got Celerity? You can move fast? Your Discipline is a bastardized version of my Discipline that lets me CONTROL MOTHERFUCKING TIME.
....repeat, as they say, ad nauseum.
Now, even if you debate the effectiveness of some of this "creep," you cannot deny that some game lines get awfully freaking crowded toward the end of the edition/revision/whatever when the developers flush everything and suddenly they want you to by a completely new set of books.
In the beginning, 3.0 had eleven character classes, with something along the lines of a dozen or so prestige classes. By the end of the run, I could name at least fifty core classes (and that's a conservative estimate, because frankly I lost track of them all) with prestige classes easily in the triple digits... and that's not counting the vast oceans of third party material.
Vampire: the Masquerade started with seven Clans. By the end of it's run, there were over forty types of vampires between all the Clans, anti-tribu, Bloodlines, etc, etc, etc. They did it again with Requiem... five Clans, sure, but there are now three or four books of Bloodlines, many with their own unique Disciplines.
I do understand, and appreciate, that RPG companies gotta sell books. From my experience, most gamers will not buy books that don't have some crunch in them. The disparity of the crunch probably comes from a variety of different sources: many freelancers who have vastly different interpretations of how things should go down, the desire to sell more books by making them seem more attractive than core, test marketing your new edition by vetting new core rule ideas prior to the Big Shakedown, or simply being so many books and years removed from the core of the game that the drift is sort of natural.
...and, maybe, just once in awhile, the developers really do feel they've "evolved" the rules into something that works better/smoother/faster.
I understand there are reasons for it, and as I said,I understand that the medium and large style companies that are in it to be businesses* do have a need to move product. Birds gotta fly, fish gotta swim. It's the cycle of the hobby-as-business world: new core, supplements, splat, splat, splat, new edition. The cycle probably isn't going anywhere anytime soon, not so long as the Big 'Uns are alive and kicking.
...now can someone release a version of the Grey Seer who can shoot nuclear missiles out of his forehead?
*Jim Raggi is trying to make a business go at this, and I would call him small press, from my comfortable Arm Chair located in the Peanut Gallery. I'd also like to say that I wish him well, and that he's sort of my gaming hero right now. I personally think there is more heart and passion in Jim's nascent catalog than in the entire WotC library...but hey, I'm just a guy who plays these games... I'm not part of the production wing of Blogaria.