If this was a singing contest then Mary, Rebecca, and John would be the ones in with a shout, most of the others would be more at home singing at their local on Friday Karaoke night.
It could be argued that this is not a singing contest. This is a contest to pick the person who will sell the most records.
Katie is terrible. Cher messed up badly during the audition process. However, the judges perhaps feel that Katie and Cher are far more marketable than those who they have booted off the show.
What they are probably trying to avoid is a situation where they have a fat/overweight/unmarketable person winning the competition, releasing 1 single which goes to No.1 and then that person goes onto completely vanish. I remember a few years ago a rather large lady won and she disappeared off the face of the planet. I'm not about the reason for this...perhaps one of your guys can tell us what happened to her and why she vanished so quickly without a trace.
Cher's voice is not good, however, to sell records you don't have to have a great voice. In fact, those with the best voices don't often sell that many records. Ultimately what sells a records is whether or not a record company are willing to back you and spend money marketing you, your music and your image.
I remember hearing an interview with a music executive and he said that it doesn't matter how good a single you have released, if a big record company want their single (even if its terrible), to get to No.1, they will market it it to high heaven and make No.1.
The point is that a singer with a good voice will probably win xfactor.
A singer with an average voice but is marketable, can sell A LOT of records.
Oh, and Susan Boyle is a VERY rare case.