See, I think Gary Gygax pegged what the old (pre-3e) thief class was for in the 1e PHB.
1.) All races (except half-orc) had unlimited advancment in thief. (the only class most did. Yes, dwarf fighters were limited to 8th, but thieves? 20th+)
2.) All races (except human) could multi-class thief with another class. Dwarves could combine it with fighter, elves with fighter, magic-user (or both), gnomes with fighter and illusionist, halflings with fighter, half-orcs with cleric and fighter, and half-elves with fighter, magic-user (or both).
3.) Put those two facts together: an elf might be an ok fighter (5-7) and a pretty decent MU (9-11) but if the took thief, they could continue to 20 (and beyond) gaining saves, thief skills, hp, etc. He also gave up little to take thief with either class: a fighter/thief could still use F/RT and OL by taking off his gauntlets and HN by removing a helm. Elfen chain allowed him to use ALL is skills (at penalty), and oh, he had backstab to augment his combat prowess (and average hp of 2-8). The MU/Thief? decent hp (2-5), thief to hit, thief skills, leather armor and thief weapons (hello shortbow & longsword!) and the spellcasting up to (potentially) 6th level spells? Sign me up. Oh, and dex synergies with both Ftr and MU. And it didn't eat a lot of XP either, thief had the lowest XP table.
Thief: Its the class demi-humans take to continue advancement once they are done being fighters and mages.
Thank the Gods of Greyhawk they made "rogue" a worthwhile class to take, rather than a half-class taken by the inexperienced and multi-classers...