I was told by a shihan that there is value in the process of testing. Preparing for a test and the event of the test itself are a chance to practice aikido under pressure, which is very different from just doing keiko all the time with no real scrutiny or consequences.
That's a good point. Most people in aikido usually train in a fairly low-pressure environment where uke always takes a nice fall and there's never any (overt) conflict allowed on the mat (this environment is often what people in aikido mean when they talk about "harmony"). For them the pressure of getting up in front of people and being scrutinized can be a challenging experience. However, if all of those testing are going to pass their 'test', then what purpose does this alleged test serve that a simple demonstration in front of the same people would not serve?
Does the potential, however remote and inconsequential, for failure increase the pressure? If so, would the process not be even more beneficial if the possibility of failure was greater (ie, if it was really a test)? What about if it were a competition where two people were being tested against each other? Wouldn't that be even more pressure? What about a real fight, on the mat but outside the bounds of training rules and customs, where the consequences of failure are much greater than just a little public embarrassment? Surely one would learn even more from such an experience than they would from a test that they are almost certainly not going to fail (and even if they do the consequences are not that great).
I would agree with your shihan that increasing the pressure and consequences to a level above what is found in the typical aikido dojo is a valuable and perhaps even essential training device. However, I don't consider the so-called tests I have seen in aikido, almost always with a very compliant uke, to be anywhere near adequate for this purpose.