it's been a long, long while since I'd written this, and I've only had a chance at a cursory review just now, but my understanding of past-me's reasoning is that Guts rolls have a slightly higher risk factor, in that a Subject that hits 0 Guts is pretty much guaranteed to die, and represents a much more direct disruptive action against the System, hence the Efficiency effect.
Guile, on the other hand, is intended for covert communication between Subjects, gathering information without raising suspicions, and yeah, is most commonly going to yield secrets as a potential reward. Granted, that is probably more implied than stated. Generally speaking, the three attributes are more like HP pools, or constantly depleting respurces than indicators of overall competence. Thematically, they are meant to be sources of risk and tension, as opposed to beneficial qualities, so the most important mechanic associated with all three of them is the fact that they get depleted, at an increasing rate, as rolls fail. One way to think of it is that the Subject has up to 18 maximum HP, but it's split three ways between three different buckets, and the current value of each container also happens to affect player character efficacy.
As such, the fact that Guts rolls are the only ones that can jeopardize efficiency away is an intentional exception. The system is pretty much constantly watching, or doing its best to approximate constant surveillance, and the subjects are very much the underdogs. Any attempt to disrupt it directly is always going to involve Guts, because every such attempt is immediately dangerous.
I hope that sort of makes sense. I'll try to reread more deeply to see if I've even got the right idea. Again, it was a while ago.