You should want to be matched with people of high ranks than you. IIRC, it helps you rank up faster. Also, I think it has more to do with your skill number than it does with your rank, which also contributes to how fast you rank.
The ranks are there as an incentive to continue playing, I guess. After you reach Lieutenant and get the achievement, you aren't really rewarded with anything else beyond that point. Maybe armor? Don't know.