People play thieves for their abilities; climbing, pickpocket, lockpicking, and in some games stealth and backstab. These are useful skills for any "adventurer" (a term I hate; it's not like you can put "Adventurer" on your resume) but there is more to a thief than just some cool skills.
Sir Dumbass surveyed the army of Orcs charging up the hill. This was going to be fun. Casually strapping his helmet on, he turned to address the motely group gathered around him.
"Drake, you take the left flank with your bow. Marge, you take the right flank with those great balls of fire you like to throw. Bob, you circle arou... Bob? Bob?"
Dumbass looks around for a moment, face reddening with anger.
"I hate thieves!"
I was sucking vomit from some guy's mouth at work the other night (don't ask) when I realized that I've never seen anyone play a thief as a thief. Sure I've seen people play thieves -- I've even played a thief myself -- but never as a thief.
For example, how many people playing a thief try their hand at con games, rigged gambling, pimping, shakedowns, burglary, cat-burglary, fencing stolen goods, forgery, grand-theft equestrian, or the like? I am aware that some of these things do get used in the course of a good adventure, or in passing as a means of entertainment when the game starts dragging. Things like forgery, cat-burglary, and fencing stolen goods (as the one trying to get rid of hot items) is a standard, which is why people play thieves.
I'd like to see someone play a thief who has to pick pockets in order to get food for the night
As I wiped bright orange chunky goo from the aforementioned guy's neck and face, I thought a bit about what I would like to see when people play thieves. I decided that I'd like to see someone play a thief who has to pick pockets in order to get food for the night as opposed to doing it for shits and giggles, or because they're bored. I'd love to have a thief spend a week researching and planning a break-in to a manor or mansion that has nothing to do with an adventure or the campaign. You know, thief stuff. How cool would it be to have a couple of thief characters come up with a bunch of con games and try to con other con artists and gamblers? Or an entire party of thieves trying to make the big score without losing all their stuff to each other or rival thieves? Or even an Animal Handler/Horse Breeder who steals some studs for a couple of nights to augment his herd without paying an arm and a leg?
Instead of playing a "thief", how about playing a prostitute who fleeces her johns and sets them up for her friends? Or a card shark who's trying to scrape together enough for a stake in the biggest poker competition in the west? Or a cat burglar by night who is a merchant during the day and only steals from his competition?
Let's stop with the "I wanna play a thief who somehow happens to get along with all the other party members, even the really honest and righteous ones like paladins and priests, who never gets in trouble with the law (unless the other party members do in the course of an adventure), and never has to worry about struggling to get enough food to live on, shelter for the night (because of course I'm with a party of adventurers who will pay for everything), warrants, rival thieves and gangs, the big score, or anything remotely thief-like".
I see true thieves every night I work. I see drug addicts and drunks, weirdoes and psychos. People who get jumped because they flashed a wad of bills at a bar or a strip club, people who get shot or stabbed because they wore the wrong color in the wrong part of town, people who have nothing and risk the last thing that they have, their freedom, for that little extra to see them through the night or until next week. Real thieves are, more often than not, desperate people going through desperate times. A night or a week in jail is free room and board for the duration of their stay.
a true thief, just like a true gambler, will know when to cut their losses and run
Occasionally you'll get people who con or steal from other people as a way of life. These people tend to be slick, smooth-talking, friendly people who by necessity move around a lot and don't have any close friends. These people are likely to travel with a group of "adventurers" (gag) as protection during their travels or as a front for less legitimate work, like slaying dragons and wiping out the entire orc population from a region. Of course, real thieves will bail as soon as things start getting hairy unless the have a sincere investment riding along with the party's future. Even then, a true thief, just like a true gambler, will know when to cut their losses and run.
And so I leave you with a few words of wisdom from the Great Gambler, Kenny Rogers:
"You got to know when to hold 'em.
Know when to fold 'em.
Know when to walk away,
And know when to run.
You never count your money,
When you're sitting at the table.
You've got time enough for countin',
When the dealin's done."