I don't consider myself especially nervous in job interviews and when taking tests and exams. But I still had a much harder time solving simple programming tasks than I would "back at my desk" with no-one watching. YMMW.
It would suck to want a job so bad you completely freeze up on simple interviewing tests :(
I understand your point but I don't really buy it. I train lifeguards as a hobby/side-job (I'm in uni right now) and pressure is the number 1 reason they give us for failing their final pratical test. I can't give a kid a permit to work as a lifeguard if the pressure of an exam makes him screw up because the real life pressure of saving somebody's life is even greater.
The exact same applies here for all jobs where you expect the engineer to work in stressful situations.
But those are in no way the same kinds of pressure. The pressure of being able to meet a release date, design incredibly safety sensitive algorithms, etc, are completely different from the social pressure of having someone evaluate work you normally do by yourself, in real time.
This kind of interviewing would work well for the kind of stress a salesman gets, not the kind of stress a software engineer gets.
In a perfect setting the interviewer wouldn't evaluate the candidate's ability to solve the problem, but his/her ability to approach it, explore it, ask questions and take steps in the appropriate direction.
Programming is not like in those hacker movies, we don't have to solve problems in 60 seconds while getting a blow job[1].
Even working in front office banking roles, there's only 1 or 2 times per year where you really have to keep your nerve and perform under genuine pressure on timescales of seconds or minutes.
The kind of pressure where 'we need to get this feature out of the door by the end of the week' is a completely different thing altogether.