Wikipedia doesn't, but let's, assume and focus on those moments when the non-dominant hand isn't necessary to cue ictuses or indicate "dynamics, phrasing, expression, and other elements".
While some conductors use both hands to indicate the beat, with the left hand mirroring the right, formal education discourages such an approach. The second hand can be used for cueing the entrances of individual players or sections, and to aid indications of dynamics, phrasing, expression, and other elements.
Wikipedia doesn't adduce the bolded phrase. Do textbooks or famous conductors that substantiate it?
Under our assumption, does the bolded phrase remain true? Why?
Don't many famous conductors mirror their hands? I wilfully picked Boulez as he's least probable to emote or gesture unnecessarily or rashly, yet even his arms mirror the beat sometimes. See his conducting L'Orchestre de Radio-Canada for the Rite of Spring on 5 Jun 1963) starting at 4:28.