If GM doesn't 'need' Honda's engineering input, why solicit it?
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Most of the industry itself, since... the beginning, looks at it more like, say; Major League Baseball. These companies are in competition with each other (duh).
When the Mets & the Phillies meet up, the Mets don't say 'Hey; you're behind 9-2. Why not use our lead-off hitter - he's @ .403 right now! Then we can have a higher aggregate game score... you just have to pay us .05% of his salary per at-bat!'
If it's legit that 'Ultium represents a milestone achievement in electrification', then let the chips fall where they may, and if that includes honda falling 'so far behind' :
¯\_(ツ)_/¯.
GM 'doesn't need' honda and doesn't own honda anything. That would include whatever minor 'economies of scale' such partnership may generate (offset by every lost sale to an Ultium honda/acura); GM doesn't need it. GM made $10 billion in net profit in a rough year last year, and they claim they'll have 30 BE models on sale 30 months from now. That'd put GM near the leading edge of BE volume (by model count).