It could also be a culture question. I worked on a trading floor and we'd make "markets" on when lunch would arrive, the cost of an airline ticket, the number of times I would injure myself in a week, etc. While there are more precise ways to ascertain a potential hire's arithmetic and estimation skills, this is more fun (especially since the interviewee's answer would usually kick off a round of bets being placed by the team on what they thought the answer was - this had the added benefit of giving us a relevant distribution to test the candidate's answer against).
It also drew out people who may not be a fit for our desk's culture of camaraderie where answers to banter along the lines of "that's personal" or "that's not relevant to my job description" would be awkward at best.
Additionally, I have heard from consulting buddies (the heathens!) that it's can good way to see how well a candidate can bullshit on little to no relevant data.
In any case, I don't think it's empirically valid to conclude someone a moron for asking brain teasers in interviews. I never asked them whereas many people much smarter than me did.
The purpose of this type of interview question is, per my argument, to get an idea of cultural fit and estimation skills. Like any measure it picks up some signal and a lot of noise, e.g. bullshitting skills. The trick is to combine measures such that the noise largely cancels out while preserving the signal.
If this were the only type of interview question used the points you brought up would be a legitimate concern. Given that it was, in my case, married in a much greater proportion with concrete questions I maintain that it generated unique, useful information.
Traders generally have, prior to entering a trade, the opportunity to think through everything thoroughly and ask for help on the bumps. There are also times when the cost of the time for analysis is so great that a gut, if rough, call is needed, e.g. holding hard drive manufacturer equity as news of Bangkok flooding breaks. Being able to ball-park figures, after issuing a disclaimer about the uncertainty of one's estimates, is also a generally useful analytic skill.
It should also be noted that part of recognising the priority of things to be analysed is the degree by which they deviate from expectations - generating many of those expectations is a form of intuitive estimation.
But, as I said, it's mostly to estimate whether this is a person we'd like to spend 10-16 hours a day near. Constant righteous indignation would probably get annoying as fast as serial bullshitting.
It also drew out people who may not be a fit for our desk's culture of camaraderie where answers to banter along the lines of "that's personal" or "that's not relevant to my job description" would be awkward at best.
Additionally, I have heard from consulting buddies (the heathens!) that it's can good way to see how well a candidate can bullshit on little to no relevant data.
In any case, I don't think it's empirically valid to conclude someone a moron for asking brain teasers in interviews. I never asked them whereas many people much smarter than me did.