I've been reading about D&D 4e character optimization and half-elves taking the Ranger's Twin Strike as their power. As I read Twin Strike, it requires two melee weapons (or a ranged one) which the Avenger and others don't want. Am I missing something?
Twin Strike is a really good at-will power.
As you get higher level, your static bonus to weapon damage tends to get bigger and bigger. Since Twin Strike doubles that bonus, it becomes stronger and stronger as you get to higher levels. At max level, it can compete with some characters' weaker encounter powers for average damage.
Furthermore, some classes (such as Avengers) like to build around getting lots of critical hits, and Twin Strike is hands-down the best at-will for this purpose (assuming you're going for single-target DPS). If you have an expanded crit range (either through re-rolls or being able to crit on lower numbers), using Twin Strike over a power that only attacks once can significantly increase your chances of getting a crit.
For those reasons, a player that chooses to focus on sustainable at-will DPS may decide that dual-wielding to take advantage of Twin Strike is worth the lower damage output on their encounter and daily powers. If your fights tend to run long (in terms of number of rounds), investing in at-wills to this extent can pay off handsomely.