[hiro]

The thing is, Ayumi's music is not centered around her voice. It's more of the overall effect of the voice and the music and the arrangement for her unlike Misia and Onitsuka. There songs are more minimalistic (compared to Ayu's) and their voices are more emphasized. Ayu is decent singer technically but the appeal of her voice lies more on the emotional impact of it.

Although she can give some bad live performances (which i think is due to her trying to force her voice to sound like someone else) she gives wonderful ones often (eg I am... in Arena Tour, Duty in Dome Tour, Surreal in Dome Tour, etc)