"From one perspective the opponent is the enemy. On the other hand there is no one who knows you more intimately, no one who challenges you more profusely or pushes you to excellence and growth so relentlessly" -The Art of Learning (Josh Waitzkin, pg 52).
"As iron sharpens iron, so one man sharpens another."- Proverbs 27:17
I was talking with a purple belt from my school yesterday, and he mentioned something that definitely got me thinking.
He said that rolling with lower belts can make you LAZY. In other words, you take them lightly, and sometimes get caught in positions that you shouldn't be in. All of a sudden you have your back taken, and THEN you decide to turn it on when they already have their arm sunk into an RNC and you are seeing stars. This is why you have to TAKE IT TO THEM at all times. This type of training is important, because you maintain a consistent state of concentration while being able to experiment with your game. This is the time to open up the game and refine moves that you aren't able to use on higher belts.
On the other hand, rolling with higher belts can be the test for your techniques. They test you and force you to stretch yourself to the limit, which will undoubtedly help you learn if you are attentive to what happens on the mat. You have to extend yourself in order to try and elevate to their level, and over time begin to improve your game.
I used to measure myself against people at my own level, but lately I've been thinking about the game in more absolute terms. Unless I can do the pass or finish the submission on a black belt, I have more work to do.
This doesn't mean that you can't learn from rolling with lower belts. It's just a different type of learning. From working with lower belts, you begin to understand the importance of fundamental techniques and foundational theory of BJJ- you feel the difference between them and people who are applying the techniques PROPERLY and begin to understand the nuances of the game. From white belt to black belt, they are using the same techniques, it's just that the black belts can do them on world class competitors.