Re: significance to testing/belt rank?
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.
He said that a single test is worth something like two months of practice in terms of aikido lessons learned.
He didn't particularly emphasize that it was the rank that was important, but the process.