Thank you to Soph Lundeberg at Soph-wise Tutoring in San Diego for calling this to my attention.
I checked the Magoosh blog post, which claimed that the GMAT would never ask test-takers to choose between “so that” and “so as to,” and something really did not sit quite right about it.” There was just no way I would have bothered to put “so that” and “so as to” head to head in a question unless I’d actually seen the GMAC do so first.
Luckily, thanks to the question index in my Sentence Correction guide (pats self on back), I managed to retrace my steps, and sure enough, there on p. 709 of the 2019 Official Guide was question 720, in which “so that” and “so as to” were pitted against one another. And yes, “so that” won. So if you’re using my GMAT SC guide and also happen to come across that blog post, know that Magoosh is wrong on this one and that question #150 is based on something the GMAC has actually tested.
I double-checked the answer key, and even though the justification for the answer was given in terms of subject consistency and some other really unnecessarily complicated grammatical mambo jumbo, I stand by the assertion that it’s easiest to think of the distinction in idiomatic terms. The other way isn’t wrong, but it makes things way more complicated than they need to be.
(Note to anyone studying for the GMAT: overcomplicating Sentence Correction grammar is a game that a lot people love to play because it makes them feel superior. Try not to fall into that trap; some of the concepts are challenging, but they’re generally nowhere near as complex as some of the “experts” at certain GMAT forums make them out to be. And, for that matter, the people at the GMAC.)
Simply put, “so that” is a cleaner and more concise than “so as to,” even if it does create a slightly longer construction overall. In contrast, “so as to” is the sort of awkward, slightly puffed up transitional phrase that weaker writers tend to use in an attempt to sound more sophisticated. That in turn makes a good construction for the GMAT to target.
Besides, “so that” is a word pair, and given the choice between an answer (or answers) that includes a correctly used word pair and one(s) that don’t, the former is basically always going to be right.
In theory, could the GMAT could create a question whose answer included a “so as to” construction? Sure: the test has been known to break its own rules on occasion. But the chances of that occurring are so small that they’re not even really worth considering.
If you’d like a slightly more advanced, logic-based explanation, you can also try thinking of it this way: “so” is a transition whose sole function is to convey cause-and-effect relationships, whereas “as” can be used to indicate that two events are occurring simultaneously, or, alternately, as a synonym for “because.”
As a result of these two definitions, constructions with “as” can create somewhat fuzzy meanings, and so the GMAT avoids using it to indicate cause-and-effect relationships. (The chance that a correct answer will include “as” used to mean “because” is for all intents and purposes zero. If you ever see a non-official question that includes this construction in a correct answer, that’s a sign that the material is not reliable.)
In contrast, “so that” has exactly one specific function, which is to indicate that x was done to produce y result.
Thus, insofar as causes and effects are concerned, “so that” (or “in order to”) wins out over a construction with “as.”