People seem to be forgetting that Jackson took A LOT of liberties with LoTR. And you know what? For good reason. The changes he made drastically helped the pacing for the medium he was working with - film. Even with Arwen, his justification of this inclusion makes sense - she replaces another character that we see very little of (and therefore don't connect with all that much), and Jackson wanted another female to balance out the INCREDIBLY male cast (seriously, we only have Arwen, Galadriel, and Eowyn). These changes make sense for a more modern audience. Even the changes in Faramir make sense to me - in the book, he isn't tempted by the ring, but the movie is framed around the idea that the ring corrupts. If Faramir weren't interested in the ring at all, the ring would be rendered powerless - which is the antithesis of what the movie was going for.
As others have stated, Legolas' inclusion is not that much of a stretch - logically, it works perfectly fine, if he does in fact appear in Mirkwood. What we need to keep in mind is that these films are simply adaptations - they don't HAVE to be completely faithful (and indeed, it would be impossible to make them completely so without making far more movies than there already are). The spirit of the source material is ever-present and revered by Jackson - that's the most important thing. If a few details are adjusted to help improve the material for a different medium, this is perfectly okay, and not the end of the world.