The Forge people don't dispute that a single person can enjoy different things.
"The Forge people" is not a well-defined group. I have heard folks who espouse precisely that - that *really*, a particular player only ever wants one agenda, and is lying to themselves and others when he or she says they want and enjoy multiple things.
A second point of vulnerability, and I think one that a lot of people (perhaps including Umbran?) have in mind, is to question the monolithic nature of the group.
I do have that in mind. I also question the monolithic nature of the individual. Yes, there are some folks who are hardcore for one agenda all the time, but many (perhaps even most) are not.
I think the idea is that you have one participant who derives pleasure from subordinating "gutsiness" to fidelity to character and setting, and another participant who derives pleasure from playing in a gutsy style. WotC seems to favour this sort of idea, with its discussion of "player types" in its DMGs (derived, at least loosely, from Robin Laws' similar ideas, I think).
Not just Robin Laws' ideas, but actual data! Data they paid a goodly sum to collect, so I'm actually kind of happy that they try to listen to it.
http://www.seankreynolds.com/rpgfiles/gaming/BreakdownOfRPGPlayers.html
The research suggests four basic agendas, rather than the Forge's three, and recognizes a mixed "middle of the road" mode as well.
You also see it in the frequently-expressed idea that combat is an alternative to, rather than a site of, characterisation and roleplaying.)
In D&D, you see such, but games like FATE demonstrate that the two need not be so separated. Of course, one would also probably argue that FATE doesn't serve gamist needs very well, the "game" part of it being rather simple.
The Forge response to this rebuttal of the "single agenda" argument, and one for which I personally have some sympathy, is that a game in which you have participants looking for different things in this way, is in some sense unstable or "second best".
And thereby making perfect the enemy of good? Or, perhaps more importantly, forgetting the social nature of the endeavor, such that being with people may be a more important part of the experience than purity of the activity.
I can analogize to a dinner party. At such a gathering, perhaps the food may be more perfectly executed if, for example, I have no individuals coming that are gluten intolerant. As soon as I have any person with any food allergy or restriction, I compromise the integrity of my menu. I'd argue that the menu is only one aspect of the overall gathering, and being too big a stickler about the food can put a damper on the other aspects.
Which is to say, if my friend is vegetarian, I'm making sure there's food for her at the darn party! Integrity of the menu can go to heck!