"But there are some of us still who go abroad for the gathering of news and the watching of our enemies, and they speak the languages of other lands. I am one. Haldir is my name."
-- Haldir in The Fellowship of the Ring, Book II, Chp. 6 "Lothlórien"
It's plausible that Elrond supplied the troops but wisely placed them under Haldir's command:
- As a part of his charge to defend Lórien, Haldir would be familiar with the terrain of nearby Rohan and Helm's Deep. The books state he was well-traveled outside of Lórien.
Rivendell however is far to the North and on the other side of the Misty Mountains.
While the force of men defending Helm's Deep are under the command of King Théoden, Elrond and Galadriel's primary purpose for sending troops is to reinforce Aragorn, Rohan's leader on the ground and the one the elves are putting their faith in to assume the vacant throne of Gondor. Haldir and his brothers escorted the Fellowship through the forest previously, so Aragorn and Haldir have some history and familiarity with each other. Maybe not a lot, but certainly more then any of the Wilhelms in shiny elf armor. Aragorn is clearly relieved to see him in the film:
A side note: when Haldir states he brings "word from Elrond of Rivendell" that doesn't mean he personally spoke with Elrond. Whoever marched the detachment from Rivendell could have served as the courier to Haldir (or Galadriel herself, since she and Elrond communicate telepathically). When he arrives at Helm's Deep, Haldir relays the message to Aragorn as the latest link in the chain, but not necessarily the only one.
So there you go: Haldir knows the land, the language and the leader. Makes perfect strategic sense for him to command.
Too bad he got his unit annihilated... :-/