I don't agree that having non-humans species puts humans in a box. I don't have a problem with non-human species having certain traits that define the species. What you actually find in such worlds is that humans still run the entire gamut in terms of character traits. I don't see any reason not to have non-human species, if you want them. Or any reason to include them if you don't want them. It all comes down to how you create your world. Either approach is valid, and either lends itself to telling complex, well thought-out stories.