A couple more things to consider...
Lemme precede this with a disclaimer. I like this tactic. It's a darn clever use of a mid-level spell. But there are problems I see with it.
First off, what kind of sword are you looking to get from this expedition? If you're hoping to snag that tasty +3 frostbrand...oops! Your Archon died as soon as it was hit (its whopping 4 hit points don't do much for it). The best you can realistically hope for is getting a masterwork weapon this way (as it won't be able to get past the +1/20 DR).
Now, maybe you're thinking that the Archon can snag that fancy +5 Platemail. So, all he needs to do is ready an action to cast the spell and make a touch attack (or maybe cast it while grappling). But oh no! Lantern Archons don't have any melee attacks!
There are ways around these problems (as you've stated: Aid, Stoneskin, etc.) But, you have to be able to communicate your instructions to it. As a DM, I'd ask the player to state these instructions exactly. And (assuming this is an evil wizard forcing a Lantern Archon into it), I'd find a loophole.
1st try:
PC: Ok, Archon, go attack the guy pictured in this illusion, but-
Archon: OK. <BLIP>
PC: D'oh!
2nd try:
PC: Ok, Archon, stay here until I say to go. Now, I want you to teleport next to the guy depicted in this illusion. But, I want you to go in and not attack him. Just sit around until he starts attacking you. Be ready to zip back here, and try to get his sword. OK, GO!
Archon: OK. <BLIP> "Hey man, don't attack me! I'm supposed to steal your sword. Just sit tight and I'll disappear in just a second."
PC: D'oh!
3rd try:
PC: OK you smartass Archon, listen. Stay here until I say Go. Now, I want you to teleport next to the guy in the illusion. Don't attack him. Don't talk to him. Allow yourself to be hit by his sword. Be ready for it, and teleport away with it when he hits you. Come back here, deliver the weapon to me, don't attack me, and then leave. Without trying to bring harm to me or my friends, or my equipment, or my horse. Don't do anything else. OK, GO!
Archon: <BLIP> "Dang, time's up! I'm goin' home!"
There are circumstances where this would be a fantastic tactic. I don't see stealing weapons or armor (or any attended equipment) as being a good use of it. However, stealing money from a vault would be good. Or snagging your friend who's being held prisoner by the evil cultists. Or going to the fridge to get you another beer, and then turning the volume on your TV up. But there are just a bunch of problems with stealing attended gear.