AD&D's demi-humans are primarily known for their multiclassing abilities. After all, what adventurer doesn't occasionally dream of casting spells from the iron fortress of plate mail armor? In an imaginary game where class defines a lion's share of one's abilities, these characters bring real versatility, albeit at a price of much slower advancement and the level limits Gygax and company thought necessary. It might come as a surprise, therefore, to suggest that single-classed thieves are the demi-human's best foot forward...
If single-classed thieves are good enough for humans, they're good enough for non-human characters, especially since the latter are objectively superior in aggregate, at least with respect to the thieving profession. But first, the elephant in the room: infravision. The ability to see in total darkness (with some conditions) alone constitutes an awesome advantage for those preferring the cover of shadow. But demi-humans come equipped with special abilities particularly well suited to their ancestries without the usual level limits.
DWARVES, customarily seen as miniature Vikings, become engineers and sappers in line with their mining civilization. Their superior constitution and saving throws allow them to better survive triggering certain deadly traps, and they enjoy generous (10-15%) bonuses to opening locks and finding/removing traps, offset by minor (5-10%) penalties where walls, stubbornly resistant to their limbs, and reading languages are concerned. Finally, their skills with mining and construction compliment this stealthy package most effectively.
.png)
ELVES, traditionally seen as archers extraordinaire, lose nothing and gain everything by choosing the thieving profession. Their high dexterities, already a prime requisite, guarantee some great missile bonuses with the short bow per Unearthed Arcana (slings otherwise), supplemented by a +1 racial bonus that includes the small swords allowed in any iteration of the rules. Backstabbing benefits accordingly. Add some solid (5-10%) bonuses to most functions and scant penalties, plus a knack for secret doors, to top it off...
It really depends on whether Unearthed Arcana is used, because this option gets Legolas, although the Player's Handbook still delivers the woodland guerillas most elves are.
HALFLINGS, small and defined by the fact, make natural thieves, enjoying constitutional boons similar to dwarves with the superior dexterity of elves. Alone among the demi-human practitioners, they enjoy broad bonuses (5-15%) to most functions, with penalties (mostly) similar to their dwarven counterparts. They also enjoy a sense of grade and direction, being appropriate to a race of burrowers, and an uncanny ability to surprise that facilitates what sneak attacks may follow. Fans of Tolkien will recognize these keen burglars.
There are others, of course, and more to say about each. But the point is clear. Single-class thievery comes naturally to demi-human characters, bestowing unlimited advancement augmented by racial abilities humans can't match barring magical intervention, ensuring the racial identities players aspire to. No streetwise thugs (not necessarily), these smaller folk leverage their natural condition towards survival in a setting of larger peoples and dangerous foes, which is something to consider before rolling yet another multiclass adventurer...