if we added pluto we would have to add over a dozen more planets. there are lots of dwarf planets in our solar system you never heard of like Sedna, Ceres, Eris, Haumea, Makemake and many more. it would be cool though to add them since Ceres is between Mars and Jupiter
But seriously, Pluto was named for the Roman God of the Dead, and yet they're still Revoking it's planetary status! You can't revolk the Roman God of the Dead's planetary status without consequences.
I think it's just for consistency, aside from the sun and Pluto they're all the same size. It's probably so you can use them without everyone having a different sized glass
COME AT ME
http://www.thinkgeek.com/product/2033/?itm=planetary_glass_set&rkgid=1656924682&cpg=ogho1&source=google_home_office&adpos=1s3&creative=51443917485&device=c&matchtype=b&network=s&gclid=CM2hgfWQj8MCFWVp7AodDl8AJw