Sup-I Greyhawk switches this up a bit. It introduces the Thief class (available to all racial types), and other races can now multiclass (i.e., dwarves can be Ftr/Thf or NPC Ftr/Clr, the new half-elves can be Ftr/Wiz or Ftr/Wiz/Clr). At this point, Elves can explicitly be one of the following class combinations: (1) Fighter/wizard, (2) Fighter/wizard/thief, (3) Thief alone, or (4) NPC-only Fighter/wizard/cleric.
Elven thieves work in all three categories at once (fighter, magic-user, and thief) unless they opt to never be anything other than in the thief category. [Sup-I, p. 5]Now, this seems like a truly odd asymmetry to me. Elves can be Ftr/wiz/thf, but they can't focus to the extent of being just Ftr/thf or Wiz/thf. They can't be a fighter or wizard alone. It seems like "fighter" and "wizard" are glued together and only come as a joint pair (unlike any other race) -- particularly odd because in Vol-1, elves could in theory just ignore one of their classes and never "switch" to using it at all. (Obviously in AD&D the options were expanded to any mix of one, two, or three of the core non-cleric classes; but by then, elves have lost anything particularly special in their multiclassing.)
So I'd like to pose the following as a question: If we look at the critical moment when Thieves are being added to the game as the 4th core class, what exactly is the OD&D elven multiclassing specialty really communicating? Are they simply "special" at picking up extra classes in general? Are they exceptionally gifted at fighting, such that they get that class for free? Are they supernaturally gifted at magic-use (wizardry), such that they get that class for free? How would the Vol-1 rule be best extrapolated to maintain the "elven theme" at the point when we add Thieves? What would fit best for the way you like to play? (See poll results here.)