Not everyone who has the hours we require is ready to perform in front of authority and may not have absorbed the skills necessary simply by osmosis. The hours are a guide and nothing more, one indication that s/he has been around long enough to learn ukemi and terminology. .
I would say the hours between testing are really the absolute minimum and there is no shame or dishonor in waiting longer if needed. There seems to be an obsession with promotion as a positive feedback mechanism for students. Sometimes this goes away as they continue in the art, sometimes not. In most cases its not as profound as I've seen in other arts (TKD as a prime example) but in some students they look at promotion as the driving force for practice. This should be discouraged.