why get a plastic saya when you can get a wooden one that looks cooler?
However the whole thing with the bo hi on a bokken. It's not exactly the best idea. It compromises the integrety of the sword. All in all, buying something like this just seems like a waste of money to me. Are you really interested in learning how to draw and sheath the sword? If so get an iaito. If you're just going to be using it like a novelty and nothing more, i highly suggest not spending so much cash, and getting one of these babies: http://www.bokkenshop.com/eng/3.html
it will last you forever and you can bash it like the mafia would a narc. Practicality over "coolness" is how i like to go.