For the same reason the other Rangers have since the series started. They morph because while a human or a humanoid alien can fight in short bursts, they could never survive a long, drawn out fight with even the Putty Patrol, let alone some hugely powerful demonic monster. Morphing evens the playing field by giving the Rangers the stamina and invulnerability to take on the monster of the week and basically walk away from a fight that would have killed an ordinary human.
Personally, I love Kat. She's so smart and good at her job that the base almost litterally fell apart without her. I'll take that kind of woman any day over some blond, prissy little rich girl with more chest than brains. Besides, even though it was only for an hour, Kat may have been the first brainiac ranger since Lost Galaxy or even clear back to Billy in MMPR.
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