Here's another quote from Kim Pensinger Witman's Wolf Trap 2006 blog from the 4th day of auditions in New York:
"I know that it’s comforting to bring an accompanist who knows you and who knows your repertoire. But please don’t bring someone who does you a disservice. It happened a few times this week. Both my colleague and I are coaches, and we know when a singer is struggling to drag his or her pianist up to tempo. (Or fighting to slow down a runaway train.) The majority of singers who bring accompanists do not fall into this category, but sadly, the singers who do make this mistake are often the very ones who can ill afford a liability like that."
I most definitely agree with Ms. Witman. However, there are those that disagree. I recall discussing this very issue with a coach of many years experience playing for opera companies and auditions. He argued that voice auditions exist primarily to show beauty of voice, suitability for hiring, and reactions under pressure. His line of reasoning continued that If a singer shows up with a pianist that they have rehearsed with, the audition shows that they have rehearsed and can work together, but not enough about a singer's suitability. Thus, even if a singer performs an audition with a pianist that is inappropriate for them, they should be able to rise above the crowd if they possess the abilities to transcend the occasion. It has been said that some German companies and agents hire second-rate audition pianists for precisely this reason--to see how the singers will react in a difficult situation.
I disagree with this contrarian line of thinking. There is simply too much at stake in a voice audition (transportation time and costs, clothing, parking, hiring a pianist, only having one shot, etc.) too accept anything less than the highest level of playing from the pianist. And I think that the quality of ensemble between singer and pianist results in a higher quality of sung performance and reflects positively to those hearing the auditions and making the casting choices (one can only hope).