First off, thanks to the all-knowing Sith'ari for his blog on the workings of a needler. Very well done. But there's another point I'd like to see discussed:
Your moving through a dimly lit hall. Several ODSTs move into formation behind you. A door swings open. Shots ring out. An ODST topples to the ground, a pair of needles embedded in his neck. You quickly dispatch the attacker, wrenching the Needler from his grasp, and turning against his allies...
Alright. Got that mental image? Now, think about the homing characteristic's of said Needler. While it was in use by the Elite, its projectiles followed the nearest non-Covenant target. But as soon as you or a fellow Marine picks up the weapon, it begins tracking all non-allied targets, while leaving friendlies unharmed. I find this odd, especially since it was doing a fine job of seeking you out only moments ago. Then again, it may just be gameplay mechanics.
My personal thoughts on the Needler's target aquisition system, is that it uses an onboard computer to match targeted enemies based on their profile. Targets with a vaguely humanoid shape are sought out, while allied profiles are marked as friendlies. Although this wouldn't explain why you have the ability to suddenly change its programming upon wielding the weapon. I'm open to suggestions. Post away.