In defense of Katherine is a very good dancer and a sweet personality. I really wish that she had been paired up with a more likeable of the pros who we complain tend get ringers though - Derek, because he dances to where his partner is featured more and to their strengths. Val or Louie would've also been a better match, though sadly if Louie came in, someone would have to be subbed out and you know the judges and producers won't choose Mark.
I have been picturing her dancing with Louie, and honestly I think had she been paired up with him - the personalities would mesh well, he would choreograph to her level and we'd be possibly accusing him of having a ringer.
Mark is just the Mark show - and yes, I feel they need to bench him a season or until he learns to give his partners more respect. (and yes, I find Derek overall a much more likeable person, and love his dancing, I just am disappointed he gets too many dancers who are seen as ringers or have dance experience).
Melissa will always be known as playing Laura Ingalls. I've personally seen her in many other roles - guest star and the like, and many of them quite serious - however, I see her in something at times, I automatically go "There's Laura Ingalls" instead of "There's Melissa Gilbert." Even though she has aged (and for the most part gracefully), people recognize her too whenever she speaks. The reason she has lasted as long as she has is because of that role, Maks being partnered with her, and overall pretty good dancing given her age.
Katherine however has two other strikes against her besides being partnered with an overall unlikeable dancer (because he does showboat) - she is not well-known unless you love opera, and she's not American and yet is dancing on an American show. People will know her if they watched the "Doctor Who" Christmas Special "A Christmas Carol" back in 2010, or have heard her sing a lot via opera.
Personally, I find Katherine a beautiful singer and a pretty good actress (the "DW" episode apparently was one of her first if not first acting role, and while fluffy, she did have to show depth in her role and did well). However, the combination of those three against a lot of familiar American actors and performers? - Dancing will only get you so far and when it comes down to people you know and enjoy watching on an American show versus an opera singer who has pretty much only one TV story to her name... people will vote the American first.