A trademark can only be defended in the face of an incursion into a protected use. If it's an "other field", then it can't be defended and the suit fails.
Fair enough and technically, I think you're right that it's not a "defense" with legal precedence (and therefore not an actual defense). But, I don't think that reality stops many companies from trying to stop the use of their brand or product in other fields because they believe they're protecting the brand.
That said, this AM General case seems to be a licensing case -- they want money for the use of the of their design. To some degree, I don't blame them since Activision is profiting from their work.
Suppose Activision sets a game in Los Angeles. A crucial mission involves a running battle down Hollywood Boulevard.
Should Graumann's Chinese Theater be entitled to money for the use of the appearance of their famous building?
To what extent should Google pay AM General, Ford, GM, Honda, Toyota, and so forth for the use of their vehicles' appearances in Streetview?
Finally: someone makes a Mail Simulator game, in which you take the role of being a first-class letter trying to get to a particular house on the other side of the continent. The role of postal vehicle is played by a https://en.wikipedia.org/wiki/Grumman_LLV
How much do they owe to Grumman, or to the USPS, or GM?
> To what extent should Google pay AM General, Ford, GM, Honda, Toyota, and so forth for the use of their vehicles' appearances in Streetview?
None. Google captured photos in public, they didn't put those vehicles there, they don't want them there and they don't add any value to streetview.
It's hard to compare that to Call of Duty where activision is intentionally modeling, texture mapping, animating and rendering someone else's design for the express purpose of adding value to their product.
I understand that the trademark itself is only protected within the same field, but it seems pretty common to defend trademarks across other fields.