Hey this could be a great thief thread. Let's keep it going.
Since there's a shortage of good NPC thieves in the game, compared to the other classes, it's a good complementary choice for players to use.
Let me make a case for the most basic of thieves- the Plain Old Thief!
Why should you use a Thief at all- ?
Well, you need to penetrate the environment. That means you need to get past doors that are locked, retrieve objects that are trapped, sneak past enemies that are too powerful, capture items from enemies, and occasionally kill a select guard or magician. Unlike the brute force approach taken by fighters - or the fireworks of magic users, Thieves are silent, and get the job done without the enemy knowing about it.
The main thing thieves have that other classes don't is the special skills. Oh, and of course, the coolness.
Now the sub kits have a variety of advantages and all that, but they suffer the limitation of less points to be deployed into the special skills. Except the Swashbuckler, unless I'm mistaken.
You can take a Plain Old Thief, and by picking areas of specialisation, make your very own "kit". As Plain Old Thieves, you'll advance in your chosen fields faster than the kitted guys( more points per level).
-The Burglar.
Burglary means breaking and entering. But most importantly, it means leaving at least mostly in one piece. Bilbo Baggins, the Halfling, was a Burglar.
Areas of specialisation would be Lockpicking, Detect Trap, and maybe Pickpocketing. At advanced levels, Burglars might opt for Detect Illusion, a very useful skill when in combat. Burglars are non-offensive thieves, generally.
If you want, you could use a Swashbuckler to do this, and grab additional fighting ability without sacrificing skills. Since they cannot backstab, you won't really need to work on stealth much. Your fighting will be up front. Swashbuckler kit makes a strong burglar.
- The Murderer.
This is a Thief that specialises in the ugly wet work of elimination. It's not an Assassin necessarily, but a character that uses the Thieves' greatest weapon- the Backstab. To focus on this area, you'll need to HIDE and SNEAK. Penetrating the environment is not as important as penetrating the enemy's kidney for this type of thief: you won't have to rely much on other skills.
Because of the low THAC0 that thieves get, You'll need to have the following- a high strength stat, an accurate bladed weapon, proficiency in SINGLE Weapon style to improve your chance for a crit hit, proficiency in the weapon class of your choice, and whatever equipment you can find that helps THAC0 and stealth.
Unlike the Stalker cousins, who just become steathier with age, Thieves have to actively choose to improve their ability. A little known fact about stabbing-
the Move Silent Skill is related to the length of time you remain invisible after coming out of hiding. With a fast weapon, it's QUITE possible to swipe, miss, and swipe again, getting off a good backstab. But your Move Silent Skill has to be well developed - over 100 at least, preferably more. The Hide in Shadows skill isn't nearly so important.
The Assassin class which is limited to just 15 points per level, is very very constrained. You'll need to be a very high level before you can branch out into other fields. On the bright side, your backstabs can get as high as times 7 as a multiplier - wow!
It's probably more effective to dual or multiclass with fighter types to become a better Murderer, than it is to select the Assassin class.