Peter Jackson is just continuing a storyteller’s tradition.
Oral storytelling predates the telling of stories by writing them in books and filming them in movies by millennia.
One can easily envision someone (Homer?) telling about a love affair. The lovers are given names, Paris and Helen. The following evening he tells of her husband’s revenge. On another evening, in front of another family’s fire, the author makes the characters royalty, one from a kingdom across the sea. And on still another, he adds a story he heard from someone else, a jealous competition among goddesses. Over a period of years, and maybe generations, you eventually get the version we read today.
The thing about oral storytelling is that the storyteller alters his/her story depending on the reaction of the audience. You embellish the parts the audience likes and dispense with, or alter, the parts they don’t like.
Neither writers nor filmmakers go from start to finished product without editing their work. J. R. R. Tolkien didn’t do so and neither did Jackson.
I would imagine that Tolkien added in and edited out a number of characters while writing his saga of Middle Earth. (Aside from creating it in the first place.) Peter Jackson is continuing the tradition—telling a story according to his personal vision to reach those he considers his audience using his chosen medium as he sees fit.
Should the tales of Middle Earth be re-made into movies again in another generation or two, the director will change interpretations of characters according to his, or her, personal vision. That director will also add or subtract characters and change emphasis according to the audience.
We may or may not like what he did, or how he did it, but by “voicing” our opinions we are continuing the age-old practice of criticism.
If you don’t like Peter Jackson’s interpretation of Tolkien, aren’t you still happy he made the film rather than not? Would we be better off having no film version of Middle Earth, other than the animated tales? Or, think about it this way, if not Jackson then who? Woody Allen? Martin Scorsese? Oliver Stone? Kevin Costner? Ridley Scott? David Lynch? Quentin Tarantino? Lynne Ramsay? Ang Lee? Who?
Personally, I like the addition. A bright, good-looking, kick-ass redhead is an asset to any action movie.