What is valuable is what we value, because if we didn't value it, we wouldn't have invented the word "valuable" to describe it.
By analogy, suppose my favourite colour is red, but I speak a language with no term for "red". So I invent "xylbiz" to refer to red things; in our language, it is pretty much a synonym for "red". All objects that are xylbiz are my favourite colour. "By definition" to some degree, since my liking red is the origin of the definition "xylbiz = red". But note that: things are not xylbiz because xylbiz is my favourite colour; they are xylbiz because of their physical characteristics. Nor is xylbiz my favourite colour because things are xylbiz; rather xylbiz is my favourite colour because that's how my mind is built.
It would, however, be fairly accurate to say that if an object is xylbiz, it is my favourite colour, and it is my favourite colour because it is xylbiz (and because of how my mind is built). It would also be accurate to say that "xylbiz" refers to red things because red is my favourite colour, but this is a statement about words, not about redness or xylbizness.
Note that if my favourite colour changed somehow, so now I like purple and invent the word "blagg" for it, things that were previously xylbiz would not become blagg, however you would notice I stop talking about "xylbiz" (actually, being human, would probably just redefine "xylbiz" to mean purple rather than define a new word).
By the way, the philosopher would probably ask "what evidence is there that we should value what mental states feel like from the inside?"
I can conceive of the following 3 main types of meaning we can pursue in life.
1. Exploring existing complexity: the natural complexity of the universe, or complexities that others created for us to explore.
2. Creating new complexity for others and ourselves to explore.
3. Hedonic pleasure: more or less direct stimulation of our pleasure centers, with wire-heading as the ultimate form.
What I'm observing in the various FAI debates is a tendency of people to shy away from wire-heading as something the FAI should do. This reluctance is generally not substantiated or clarified with anything other than "clearly, this isn't what we want". This is not, however, clear to me at all.
The utility we get from exploration and creation is an enjoyable mental process that comes with these activities. Once an FAI can rewire our brains at will, we do not need to perform actual exploration or creation to experience this enjoyment. Instead, the enjoyment we get from exploration and creation becomes just another form of pleasure that can be stimulated directly.
If you are a utilitarian, and you believe in shut-up-and-multiply, then the correct thing for the FAI to do is to use up all available resources so as to maximize the number of beings, and then induce a state of permanent and ultimate enjoyment in every one of them. This enjoyment could be of any type - it could be explorative or creative or hedonic enjoyment as we know it. The most energy efficient way to create any kind of enjoyment, however, is to stimulate the brain-equivalent directly. Therefore, the greatest utility will be achieved by wire-heading. Everything else falls short of that.
What I don't quite understand is why everyone thinks that this would be such a horrible outcome. As far as I can tell, these seem to be cached emotions that are suitable for our world, but not for the world of FAI. In our world, we truly do need to constantly explore and create, or else we will suffer the consequences of not mastering our environment. In a world where FAI exists, there is no longer a point, nor even a possibility, of mastering our environment. The FAI masters our environment for us, and there is no longer a reason to avoid hedonic pleasure. It is no longer a trap.
Since the FAI can sustain us in safety until the universe goes poof, there is no reason for everyone not to experience ultimate enjoyment in the meanwhile. In fact, I can hardly tell this apart from the concept of a Christian Heaven, which appears to be a place where Christians very much want to get.
If you don't want to be "reduced" to an eternal state of bliss, that's tough luck. The alternative would be for the FAI to create an environment for you to play in, consuming precious resources that could sustain more creatures in a permanently blissful state. But don't worry; you won't need to feel bad for long. The FAI can simply modify your preferences so you want an eternally blissful state.
Welcome to Heaven.