Here's a great article describing it.
http://www.guildwarsguru.com/content...nics-id674.php
The short answer is the ONLY benefit of having a weapon with a lower requirement is more flexibility in your build - that is, allocating attribute points to attributes other than your weapon skill.
It doesn't matter whether you have a req 7 sword or a req 12 sword, your damage and critical hit chances will be the same as long as you meet the requirement.
That being said, it is preferable to put as many attribute points into your weapon attribute as possible, due to the increase in damage and increased chance of a critical hit, as described in the article linked above.