At my dojo, if you stay at a low belt indefinitely long you will not learn some of the more advanced stuff, particularly katas. Usually, you go up a belt and then you learn the kata for the next belt. Although students sometimes do learn more advanced katas you are not going to know katas that are that much more advanced. A white belt is not going to know brown belt katas.PHOTONGUY SAID: ↑
Well the thing is, if you do train at a dojo that has ranks, if you stay at a low rank for too long it can cause complications. With higher ranks its not such a problem but staying at low ranks overly long can cause complications with the training.
Why? I guess it depends on the school. At my instructors commercial or open dojo he has an age requirement for 1st dan of 16. So students if they start at 6 yrs old might be in the school for 10 years before the test for shodan. It doesn't mean their training was hampered just that by then they are really good 1st dans. He has requirements and you bend to that requirement period, one of them being age for 1st dan.
Also, most of the drills we do are partner drills where you work with a partner in the class. Most of the time you will work with somebody who is the same or close to the same rank as you. Sometimes a much higher ranking student will partner up with a much lower ranking student in which case the higher ranking student will help the lower ranking student but most of the time you work with somebody who is the same or around the same rank as you. This can cause complications if your skill level far exceeds your belt rank.
So this is why its not a good idea to stay at low ranks for too long. At higher ranks its not such a problem but at lower ranks it can cause those complications.