I don't disagree. My point, however, is that when Joe Midlist's agent goes in to negotiate, Joe's agent is much more likely to try to negotiate the advance up from, say, $5,000 to $6,000, rather than trying to get an extra point on the royalties. X dollars now is almost always more appealing than the possibility of X+/-? dollars later, especially since everyone involved knows how low the odds are that the book will earn out even at that level of advance.
As to the raw materials comparison, I'm not sure it's apt. The arts almost always puts the work ahead of the reward, except for back in the days of arts patronage. I admit that my perspective may be skewed by the fact that I write fiction, rather than non-fiction or technical manuals, though.