I think Mary's smokey vocals add some to the track and I think if ALL three girls had some sort of lead presence, it would have been even better. and it would have helped revitalize the group and doing something different

Mary taking the first part of the verse, which is in the lower register, works. but she doesn't have the vocal gymnastics to really add the power the track needed.

And frankly the lyrics are silly and so I think the song never was really destined for masterpiece or mega hit status. it's just not good enough of a song.