Since I can't put it any better than Ian, I'll just quote him.
Ian Hurst wrote:
2. "I feel that physical technique is a very small part of the grand scheme of Aikido" - I nominate this particular phrase as the one I most disagree with to date.
Whether you're in aikido for your spiritual advancement or (as many of us are) in it for the martial aspect, the physical display of your art is what you should be measured by, not how well you (for example) can bake a cake for the entire dojo. The physical side is the direct expression you have for your understanding of aikido and should be the primary way others can measure your aikido.
While I take Don's point regarding aikido vs physical/mental limits and have modified gradings with this in mind, there is also a limit. High grade in aikido must represent a strong ability in the martial art , after all that is what aikido is, not a social club or a church.