User blog comment:James12708/Yahg in multiplayer?/@comment-4967682-20120506215130/@comment-4967682-20120506220900

Because one, we already have the krogan, two, they'd be really sluggish to play, and three, it would make absolutely no sense story-wise. The vorcha and geth make little sense as it is, but adding the yahg (who are explicitly stated to be quarantined to their home and left alone by the Reapers) would make even less sense. Even if the yahg happen to be those salarian test subjects, it would make no sense.

Plus, consider this: they're smarter than salarians, much stronger than krogan, as adaptable as humand, and can accurately wield Revenant Light Machine Guns one handed (their weight capacity would be close to infinite). They'd be very overpowered.