Novelizations used to be for fans who wanted that extra insight into the story, unburdened by the limited running time of a movie and it's pacing - they dwelled on character's thoughts and explained things that couldn't be visually or weren't absolutely necessary or crucial to explain in the film, and so they're often almost like the Bible of the film, along the script. Let me underline that none of the answers are my own opinions, they are an intent of the filmmakers and official sources will be quoted when necessary. And have in mind, this entire site is just about Cameronverse only.
Let's continue with "Why Does The T-1000 Revert to the Robert Patrick look?"
As far as storytelling purpose, it is done of course so the audience has an identifiable villain. But originally, Cameron did not envision the T-1000 to have a default design in the story, and it was Stan Winston, the special effects legend, that suggested to Cameron that the T-1000 should have an identifiable face. Excerpt below from "Winston Effect" book
However, as with everything in Cameron's science fiction world, it does have its in-universe explanation, suggested on screen, flat out stated in the novelization. To execute any kind of morph, the T-1000 uses up considerable amount of energy, so just like the first Terminator (in the novelization for the first film), the T-1000 is also trying to conserve his energy and reverts back to the default look. Excerpt from T2 novelization
The film suggests it by showing that the T-1000's nanobots cannot multitask - the T-1000 only heals after it accumulates damage (although there is also a real life reason for that) . Cinefex magazine #47:
That's why the T-1000 does not heal its wounds during the Pescadero car chase, when he's engaged in action. He does morph into the Officer look while exiting the elevator, but he initiated the morph when in static startup position and completed the morph before reaching the full running speed.
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