I don't know why we don't use roses as bonsai more often. They are hardy, good in most climates, and its possible to find really old hedge material.
I saw one in someones yard the other day. it was awesome! good nebari, old trunk with taper, lil yellow flowers. I knocked on the door, but no one was home.
Shinji Susuki says his favorite tree is a rose that his old master (Kimura's master also), Hamano gave him. So they are being used in the Bonsai motherland, Japan.
We use many species of trees that are close relatives for bonsai culture, like crab apple and hawthorn.