The first time I saw a Thief, I thought "Wow! 15 prowess!" Then I read the text and discovered that instead of harming a character, the Thief would steal an item from the company, but your opponent would choose which one. With 15 prowess, some companys will simply give up an item instead of trying to beat the Thief.
Because most (if not all) companys start with two minor items, Thief wouldn't be of much value, unless you use three of them (or the company has already use the minor items, if they were one-shot, like Miruvor).
The Thief is keyable to border-lands and border-holds, and that's, at least, good. There are few important border-lands on the map (Ladies and Gentlemen, have any of you never been robbed, slayed or ambushed in Anduin Vales?). The only region that's free of border-lands is west of the Misty Mountains, but the decks around Rivendell usually go to Bree and the Old Forest. The Under-deeps decks are going to cause headaches in a Thief hazard deck, because they tend to stay a couple of turns before going back to the surface, and when they do, they usually go right to a Haven to store items and to heal. In that case, holding the Thiefs in your hand isn't worthwhile.
So, Thief need some enhancers. Until the release of MEDM, the only way to increase the Thief's strikes was playing Hoarmurath of Dir, which would work for only one Thief unless you recycle it. But with Rank Upon Rank (#353), you can easily increase the number of strikes of the other Thiefs too and use Hoarmurath to give a third strike. Also, Indur Dawndeath (#62) is a good option to steal items from wounded characters (when they need it most). I suggest using Were-worm (#199) to cover Wilderness, because your adversary will try to avoid border-lands after the second Thief, probably using some extra wildernesses. You'll also need some recycle ability, like Uvatha the Horseman (#71) and Mouth of Sauron (and From the Pits of Angband (#223) for Were-worm) to return the Thiefs back to your hand, or An Unexpected Outpost (#279), to recycle the Thiefs and the others recyclers or Thrice Outnumbered (#494), but Thrice works better in an all men hazard strategy. The other stealers, like Brigands, Pick-pocktes or agents, are either too weak or too restricted to be effective.
If your adversary starts to store items, pick some Neither so Ancient nor Potent (#475) from the sideboard and wait until he calls the council. A warning: if you know your opponent is going to use hobbits, you will need to prepare a big ambush (with Two or Three Tribes Present (#292)) in a border-hold - first play some Assassins (#115 and #522) to break down resistence, maybe an Abductor (#20) and then the Thiefs (personally, I love to yank Torque of Hues off an all Hobbit company). Such strategy will require that you hold some cards in your hand, so use Pallando, The Emerald of the Mariner (#162), The Book of Mazarbul (#467) or Elven-Kings.
If you really want to use Thief, you must build your hazard deck (or part of it) around it. However, it is quite satisfying to leave your opponent wondering how the Thief got Frodo's Mithril-coat while he was wearing it.
Original card review taken from : http://fan.theonering.net/morgulrats/
With the authorization of the webmaster.
The reviewing team consisted of Gwaihir (Chris Farrell), Gimli (Nathan Bruinooge), Ohtar (Charles E. Bouldin, Esq.), Radagast (James Kight), Joshua B. Grace (Beorn), Martijn Steultjens (Fram Frumgarson), Jason Klank (Saruman) and Jeffery Dobberpuhl (Wormtongue)