If you're going to let people play your game before it's released, you may as well get guys and girls who have some reference point as to how a battle rifle actually works. Their feedback at this stage of development would be invaluable.
In what was either the afore-mentioned beta testing idea or a Christmas present to US soldiers in Iraq (admittedly, most probably the latter), Bungie flew some early-build copies of Halo 3 out so American troops not able to get home for the holidays could at least get some kind of Christmas cheer.
And what's best, we now have some screenshots (blurry shots are better than no shots) after the leap.