The standard SCIM traditional chinese input does not have a good mandarin pinyin option. It is possible under the simplified chinese Smart Pinyin if you click the 中 button 3 times, but the suggested characters/phrases are no longer very helpful or good. Plus it is very annoying to always have to click the button 3 times. Under the Traditional Chinese method list, they should have their own mandarin pinyin option, not having to use the one in the simplified list.
Some people say that only Taiwan uses traditional, and most people there know/use ZhuYin (bopomofo). However, this is not acceptable. In some major cities, like Taipei, they have switched to teaching children pinyin.
So, I think SCIM for traditional pinyin users should be made as good as the Smart Pinyin for simplified. Windows and Mac are both very good at this already. I hope SCIM can catch up very soon.