Everything I read says that when you attain Shodan, you basically know enough to know that you don't know much. So, I don't see why there needs to be six ranks below the rank where you know that you know nothing.
You seem to be a straightforward, speak-your-mind person, so while I know it's tactless, I'll be as straightforward as I can be: you need to read more, practice more, and trust your sensei more.
Testing in aikido serves many purposes. You can't understand them all as a beginner. Many people have mentioned many of them in this thread, so I won't repeat them. There are many other reasons to test, I believe, of which I'm still not aware.
The "shodan is where you realize you don't know anything" line means this to me: I have learned all the basic skills of aikido, and now it is my task to more fully understand and perfect them. Much in the same way that you learn the basic skills of writing and then write to perfect those skills. Just knowing the "rules" for composing an essay doesn't mean I am proficient at it.
Just testing on shihonage at 6th kyu doesn't mean that I have a master's level of proficiency.
So why test?
In addition to the many ideas already shared, here are some of mine:
1. To more clearly display to my sensei my understanding and skill in technique.
2. To show myself how well I maintain composure under a stressful setting.
3. To show students who are my juniors the level of proficiency to which they will be expected to rise.
4. To show students who are my seniors how well or poorly they have helped to prepare me. (This may have been mentioned already, but I couldn't find it on subsequent readings of the thread to be sure)
5. To maintain tradition.
6. Because it's a reasonable request, and it's polite to do as I'm asked by my teacher.
That being said, I find testing personally pointless. It's not why I practice aikido. I was at second kyu for several years, because I didn't ask to test, and nobody else really paid attention to my hours. Hreha Sensei was at the dojo for a seminar, watched me, looked at the rank board, shook his head and walked away. The next day he promoted me to 1st kyu without testing me.
I would likely have stayed there, too, except that it was pointed out to me that my children's class students needed to see that I was continuing my aikido education, and that for them, the easiest way to do that was for me to go forward and test for shodan.
I'm glad I did. I proved to myself that I could do it (yes I had doubts). It was one of the most mentally and physically challenging things I have done.
The shodan test is not "THE" test. It's one in a series that shows stages of progression. It's the one OF the series that tests on ALL the required techniques. Future tests will help me to evaluate my level of refinement.
And I'll take them when asked to. It's no big deal.
(Yikes. That's a lot longer than I set out to write.)