I suppose it comes down to how you perceive magic - as a science or an art.
Obviously, if you consider magic a science, as it seems to have some very strict rules in the Sixth World (like the correlation between spell power and drain), then the scientific approach obviously has the upper hand. Basically, it is the difference between a talented car mechanic and an automobile engineer. Even though the mechanic can probably do a good job on a car, he cannot measure with the engineer, who really understands how a car works, and what he can squeeze out of it. In this case, hermetics gain the upper hand, as their better understanding would give them an advantage in the end.
On the other hand, if you consider magic as art, then the shamans are right, as in this case creativity, intuition and improvisation have much higher weight than the scientific approach. It's like with musicians - you can have some very good instrumentalists, but as long as they don't have that passion, they don't sound as good, even if they have a much better technique.
But in the end, this is the good thing about magic, that it can be considered both a science and an art, which gives a lot of room for interpretation, and of course, mystery (never a bad thing, if you ask me). It is quite similar with the Matrix and the difference between hackers and technomancers. The former approach the Matrix in a more scientific manner, while the latter do it as artists, and they never seem to be in such big conflict.
And to mention a few words on the original topic, as we diverged quite a lot in the last few posts, one explanation for the mana fluctuation could be that it is simply a natural order of things. I know this not quite an explanation, but the universe itself seems to contain quite a lot of different cycles, even if not regular ones. In fact, this gives another line of thought - what if the length of the cycle (in other words the Worlds) is not the same? Of course, Ehran claims this is so, but what if he doesn't really have a clue? This could well mean that the Horrors could much sooner (or later). I guess some of the Great Dragons might have an idea, but since none of them is talking about it, a simple explanation could be that they are as clueless about it as everyone else, but would never admit it.