You're missing the point - the actual callback itself, is about an attempt for the lawyers to derive how malleable you are. How amorphous. They want someone book smart who doesn't intellectualize.
From this, we gain a new understanding of the mystique surrounding the process - "we won't tell you what we want, because it doesn't sound good" -- It's NOT just about long hours, it's NOT just about hard work -- it's about something quite more sinister... at least at the V10 firms. Other firms try to value the individual more, or at least they say they do. Perhaps this is a disinformation tactic though. If you show too much of a brain, a think for yourself attitude, they don't see the cost of a callback as too much of an expense since you might be back one day when you've got a client base. Then you're free to think however the hell you want.
This is BS. I got an offer at a V10 firm where, during the call back, I told a senior litigation partner that I thought that the PSLRA was a bad law and that securities fraud cases should almost never be thrown out at the pleadings. He said that once I work on a couple of those cases I will change my mind. What I said was fine because even though I obviously had opinions, I presumably did not come off as somebody who is so wedded to those opinions that it would interfere with my ability to provide zealous advocacy to the firm's clients. The reason you get in trouble for talking about work/life balance and pro bono is not because they want people who are malleable but because they want people who are actually interested in doing the job they are hiring you for. If all you want to do is leave the office at 6 o clock and handle public interest cases, there are plenty of places where you can do that, but why on earth are you interviewing at a corporate firm? Because you are willing to sacrifice your desire to do that full time in order to get more money. I have news for you, the guy who is selling out his dream and going to the firm for the money does not come off looking better than the guy who actually really wants to do M&A. As for the fact that the firm is worried that you might have qualms about representing their clients - you have no business representing a client if your qualms would interfere with your ability to advocate for them.