I've always thought that because (most of us) don't do competition, gradings are the one place we have to see how our technique fares with the nerves/adrenalin pumping.
This is what our gradings are for. Sensei wouldn't put you up if he didn't know you were capable of doing it. The test is to see if he can "take your center". He wants to see if you can overcome the nerves and carry on.
Recently we had two people in our dojo promoted to nikyu without a formal grading. The reason they were given their promotions was because they both led aiki-taiso (warm up exercises) at our national summer camp. Sensei and the other high ranks watched them and all agreed that even though they were nervous they got it under control and did a fine job. They had shown they could perform under pressure therefore no need for the test.