Yes, everyone knows Tauriel didn't exist in the book, let alone became romantically involved with a dwarf. However much you may despise Jackson's inclusion of characters and sub-plots not present in the original novel, they do add substance. Okay, so you could argue that the relationship between Kili and elven admirer is utterly pointless and only serves to pad out a bloated running time, but romance sells, and the series in general is rather devoid of it in any personal manner. There are numerous examples of a generalised sense of loving for one another in both The Hobbit and the Rings trilogy, but Tauriel and Kili's relationship provides a personalised example in a similar manner to Aragorn and Arwen's. It's also another example of inter-species relationships, only this time between a dwarf and an elf. Quick, someone re-write that Percy Sledge song.