Within one dojo this is common accepted policy. It does however not prevent people to do exams/test at a different place, masquerading as mister wonderfull aikidoka. (For those wondering, yes this does happen!!)
Okay... mr. Knoops... maybe I've lived in a way too safe and beautiful world
welcome cruel and harsh reality. How does one prevent them anyway? How is it possible for a different sensei, one who doesn't know your character to judge it? Of rumors he heard? Of 15 minutes of "serious talk" with the original sensei? I don't think so. The only thing that can be taken in consideration is technique. Because it can be measured more or less. And I personally think that all that Aikido stands for (what You called "whole Aikido") it is in the techniques as well.
Could you give an example of someone who has exellent technique but a crappy character? I mean lazyness should show in techniques... arrogance as well... pride - yup, he won't take good ukemi. Etc etc. And when someone is just a good actor enough to hide all those bad things during the test then... what can we do? I guess nothing.
P.S. Just for the record - in Estonia there is such policy that normally anyone can take a kyu test which is given by the head instructor of Estonia (who hardly "knows" all the students) but the student has to be permitted to the exam by his own sensei (who knows whatever he knows).