There are definitely people who are capable of solving problems but whose performance drops appreciably under pressure. Now, you may not want to hire those people, but that doesn't mean it has anything to do with external motivation or making excuses, and you can keep your contempt for yourself.
There are many different kinds of pressure. The pressure in an interview is very different from the kind of pressure experienced when the database is down.
For example, I have no problem at all with on-site interviews but I choke pretty easily during phone interviews. I also have no problem with public speaking, but I have trouble engaging people I don't know in one-on-one conversations.
As an employer, I want employees who can solve problems whether they are under pressure or not. You really want an employee working for you, or even a team member in your group who can't work under pressure when your site is down and they are the only one who can fix it? If you can't work under pressure, you can't work. Or maybe you can work, but I'd rather work with someone who can work under pressure or who can't. That's my point. And I believe it has a lot to do with external motivation. I apologize for the contempt.
Under pressure and "in a strange environment" are two different things. You could put interview candidates in SCUBA gear and take them down 5 fathoms to see how they'd do under pressure, but that's also completely unrelated to predicting their actual job performance.
We don't ask trivia questions (what's the order of arguments to fputs, list all the overloads of foo in class bar) because knowing that (or anything else that's a 5 second lookup or context-sensitive help away) doesn't predict how someone will actually perform on the job.
Can any of us code as effectively on a whiteboard as we could in our favorite editor? I'd expect not, and if you were equally able on a whiteboard as in an editor, I'd wager that it's because you're not particularly good at either.
One thing I learned a while back at one interview (or rather, I just worked it out after experiencing it a few times) is that I find it much harder to think/code on a whiteboard. I don't know what it is, but it seems much easier for me to sit down with a pen and paper and work things out.
I mean, still coding without a computer - but it's something about standing up at a whiteboard. Maybe it's normally not so bad for me, but with the pressure of an interview, it's bad enough to give me problems.
I don't think it is only about pressure. I can think up a solution to a problem in my head, but to really work it out I have to be behind my computer and start typing. I almost never get something right in 1 try.
I think my style is more trial and error, I make something and see if it works, if not I continue. So of course, a lot of great programmers actually are able to do that, but I think there are also a lot of people who work like I do.
There are definitely people who are capable of solving problems but whose performance drops appreciably under pressure. Now, you may not want to hire those people, but that doesn't mean it has anything to do with external motivation or making excuses, and you can keep your contempt for yourself.