They are a pop band, like Mcfly, Scouting For Girls, Rascall Flats, No Doubt, Panic At The Disco and Avril Lavinge and some of Maroon 5's, Linkin Park, Hoobastank and Fall Out Boy's stuff. If they were real rock stars they would more stylistic and emotional in their delivery and they wouldn't try to iron out all their flaws. They think perfection is more important than being emotional.
Plus her voice is just ordinary, similar vein to Toni Braxton type voice where it's got some quality and some people love it, but she doesn't overpower a pop song like someone who has an unusual voice (like Alanness Morressette) or a great voice would. But then for RnB voices you do need really great voices or very unsual timbres (rather than just a pretty sweet voice or a slightly grating voice like Rihanna).