If you look on the Vocaloid site, it says Miku's good as a backing singer. Actually, Miku sound fairly powerful in this song. She's capable of singing weaker than this. Besides, isn't it cute? At the end of Dec 2007, a new voice is being released. This voice will be able to sound much more powerful than Miku.
VOCALOID no saito wo mitara, miku ha shien no kashu koto wo shimeshimasu. jitsu wa, kono uta ni yori ippai yowai koe ni utau koto ga dekiru. 2007-12 no saigo ni, atarashii koe ga dasaremasu. kono koe ha miku yori zutto tsuyoi koe ni utau koto ga dekiru. (gomen, nihongo ga muzukashii...)