She is a much better employee of W&H than Angel.
I'm sure that Hamilton thinks so.
But seriously, for all Harmony's nice-ness she has no moral core, she is evil even if she is friendly and bubbly. She drinks pig's blood because she will be killed if she doesn't, not because she cares about people and is taking a moral stand.
By betraying Angel to the senior partners she is betraying
everyone of the gang, including Spike and Lorne who at times treated her with respect (well, Lorne did I guess). Did Harmony care? Nope, she just played the victim and whined about not being trusted and included in the plan, even though (ironically) she was.
Honestly, in the end, Angel showed Harmony respect, admitted that she was good at her job and even useful in the fight against a great evil, even if that was never in her nature.
Btw- I truly dislike Harmony in the comics, where the writing just gave up on portraying vampires with any consistancy in order to tell the story they wanted. 'Harmony's Rules' are a joke.