Training with people who are better than you is important. It teaches humility; you ain't the be-all, end-all of martial arts (yet). It teaches confidence; when you can take what they dish out, and even give a little back in return, it feels like you're on the right path. It lets you throw your techniques with all the speed, power, and precision that you can muster, knowing that your uke can handle it without being hurt; this is important because you need to know how it feels to throw that technique with all your might and main. It gives you a sense of place and purpose, and it builds confidence yourself and others in your dojo.
It is also important to train with people who are DIFFERENT than you. By that, I mean that heavy people need to train with thinner people. Tall people need to train with short people. Men need to train with women. When possible, different arts need to train together (safely, not in a 'my way is better, here let me prove it' manner). Everyone uses their art and their personal attributes to their own best advantage.
Proverbs 27:6 Trustworthy are the blows of a friend, dangerous, the kisses of an enemy.
Proverbs 27:17 Iron is sharpened by iron; one person sharpens another