Utility can't be one dimensional and it probably isn't linear.
In other words, we would have to treat most situations as unique and then try to find patterns and then the whole rationalism thing goes out of the window.
Only spherical-rationalism-in-a-vacuum goes out the window.
Unfortunately, and this goes back to my previous point, lots of people (I'm not immune!) mistake taking the first baby step for climbing the whole mountain.
In other words, we would have to treat most situations as unique and then try to find patterns and then the whole rationalism thing goes out of the window.