I agree with your first paragraph, but I think your second is just making the same mistake again. Why should our utility function be a product any more than it should be a sum? Why should it be mathematically elegant at all when nothing else about humans is?

^ What he said.

Though note that utility is often fungible in small quantities, since I can always trade one good for another at market value (this fails when the quantities involved are large enough that the market can't absorb them without affecting price, or if the object in question is difficult to liquidate, e.g. time or knowledge).

...some agents have 'fungible' utility, where one unit of X has the same value regardless of how much you already have or how much of anything else you have...

What about the human desire for positive bodily sensations? Given what we currently know about physics, it should be much more efficient to cause them unconditionally than to realize them as a result of some actual achievement. Humans value such fictitious sensations, see movies or daydreams. So the value of such sensations is non-negligible. If we can create them effectively enough to outweigh the utility we assign to their natural realization, then isn't it rational to choose to indulge into unconditional satisfaction?

If only one of your values can be realized an unlimited times, then it only needs to yield one unit of utility per realization to outweigh all other values, as long as its realization is cost effective enough. Because as far as I know, the utility from realizing that one value is no different than the utility you can earn from any of your values, all that counts is the amount of utility you expect.

Just like a fine dinner of many very good dishes is better than only the ice cream you like the most.

Yes, but take for example companies. Companies are economic entities that resemble rational utility maximizer's much better than humans. Most companies specialize on producing one product or a narrow set of products. How can this be explained given that companies are controlled by humans for humans? It seems that adopting profit maximization leads to specialization which leads to simplistic values.

A larger plethora of values can be of the greatest value.

The large plethora of values mainly seems to be a result of human culture, a meme complex that is the effect of a society of irrational agents. Evolution only equipped us with a few drives. Likewise does rational utility maximization not favor the treatment of rare diseases in cute kittens. Such values are only being pursued because of agents who do not subscribe to rational utility maximization.

Can you imagine a society of perfectly rational utility maximizer's that, among other things, play World of Warcraft, lotteries and save frogs from being overrun by cars?

for example I would regard hyperbolic discounting and diminishing returns both as an example of 2D utility.

The OP was triggered by thinking about wireheading and the reasoning that leads some people to adopt it as desirable, or who don't perceive it as something that is obviously undesirable for humans. Here the first question that comes to my mind is if the adoption of expected utility maximization could reduce the complexity of human values to a narrow set, such as the maximization of desirable bodily sensations.

I reject wireheading myself. But on what basis? To have a consistent utility function one needs to know how to assign utility to new goals one might encounter. If utility is not objectively grounded in some physical fact, e.g. bodily sensations or qualia, then how could we judge if a new goal does outweigh some of our previous goals? For example, if a stone age hunter gatherer learns about the opera, how can he adapt its utility function to such a new goal that can not be defined in terms of previous goals? Should he assign some arbitrary amount of utility to it based on naive introspection? That seems equal to having no utility-function at all, or will at least lead to some serious problems. Here I suspect that a valid measure could be the amount of desirable bodily sensations that are expected as a result of taking a certain action.

If we were to measure utility in units of bodily sensations, then by maximizing utility we maximize bodily sensations. Consequently, as expected utility maximizer's we should assign most utility to universes where we experience the largest amount of desirable bodily sensations. This might lead to the reduction of complex values to the one action that yields the most utility, wireheading. I don't agree with this of course, just one line of thought to explain where I am coming from.

Now measuring utility in terms of bodily sensations might lead any human utility maximizer to abandon his complex values. But it will also exclude a lot of human values by the very unit in which utility is grounded. For example, humans care about other humans. Some humans even believe that we should not apply discounting to human well-being and that the mitigation of human suffering never hits diminishing returns. This is incompatible with the maximization of bodily sensations because you can't expect your positive feelings to increase linearly with the number of people you help (at least as long as you don't suspect that universal altruism isn't instrumental to your own long-term well-being etc.). But the example of where an objective grounding of utility in bodily sensations breaks down is also an example of another reduction of complex values to a narrow set of values, i.e. human well-being. If you measure utility in the number of beings you save, your complex values are again outweighed by a single goal, saving people.