MBLs and other omnis do not work well that close to the wall in my opinion. Let me explain:
In general, early-arriving reflections are undesirable. The can cause coloration, image-skewing, and degradation of clarity. On the other hand, late-arriving reflections are beneficial as they impart a sense of liveliness and richness without the negative side-effects of early reflections.
The transition between detrimental and beneficial reflections is about 10 milliseconds. In other words, ideally you want the extra reverberant energy from an omni (or dipole or bipole) to arrive at least 10 milliseconds later than the first-arrival sound. This is a fuzzy transition zone, rather than a clear line in the sand.
Sound travels a little over one foot per millisecond, so 10 milliseconds corresponds to a path length difference of about 11 feet.
In my opinon, given Yetis' placement constraints fairly directional speakers make sense as a way to avoid detrimental early reflections. In this I agree with Triode's comments. Placement very close to the walls can cause overemphasis of the bass region, so that might be a factor as well.