I did anticipate there would be some questions along the lines of
"why can't another class fill the scout role?". I'll give you one answer in a bit that is technical and probably unsatisfying, but the short answer is of course that you are correct---another class can serve as the party's scout.
Obvious choices are ranger and/or races like hobbits and elves who gain (in 1e) advantages in surprise when they are out in front of the party and not in armor. That's really the key though...to be sneaky you give up armor. Which is why that role is somewhat against grain with a fighter.
"What about a fighter with elvish boots, invisibility, etc. etc.?"
Sure. You can jazz things up and get there as well---but then I think you are again playing at min/max optimization and not really embracing the spirit of the game. All that magic just to make the perfect scout? (That said, with my kids, as soon as our magic-user had
invisibility, the thief was always asking to be made invisible before dangerous excursions---heck, the
whole party wanted to travel around invisible "holding hands" the whole time if they could get away with it. It was a constant refrain because stealth is golden in a dangerous world.)
Taking a step back for a second. The point of what I wrote was to address the following:
Why I never personally experienced issues with thieves in OD&D/1e---e.g. don't they suck? I say NO because...
- They fulfill the role of scout for the party -- crucial if "balanced encounters" mollycoddling, 5 room dungeon paradigm, and D&D as equal-participation-combat-porn have not been invented yet
- They don't solely rely on the success of their initially low ability scores to function usefully---those skills are icing, and kick in more at higher levels when it matters more (i.e. when greater risks are taken & character death is more devastating)
- They aren't expected to be heavily involved in combat, using their backstab strategically.
Also, for some context, when I started playing D&D after the release of the Holmes Basic (B1) edition --- there weren't too many class options (and we weren't experienced or mathematically inclined) so we took things at face value. For the role of scout/thief, we simply played thieves.
After the 1e AD&D PHB came out, that's when the min/max-ing started. Really. All of a sudden EVERYONE was a ranger/paladin/bard or multi-class fighter/magic-user/cleric. I mean it just got silly and stupid. There was a lot of talk about Monte Haul DM's, but truthfully it was player greed that ruined the game. No one was satisfied with just being average and playing anymore. They sought to "win" through character builds. This was 1978! This was made worse because the DMG wasn't out yet, and the Basic rules didn't include the checks-and-balances against power-creep---which, after they did arrive, were mostly ignored.
Fortunately, when I was just about done with the stupidity of D&D (as practiced by middle-schoolers), I bumped into some older kids playing OD&D (LBB+). Our DM did not let players have their way. There were rules and structure and everything was a challenge. We weren't allowed to peek behind the curtain, and had to take things at face value. Multi-classing was allowed, but prohibitively expensive. That's the D&D that kept me engaged for a decade as a player. That's the D&D I passed on to my kids as a DM that we've played for the last decade. That (IMO) is the D&D that lasts and is good.
Sure, a ranger is a natural scout---but we were almost never were lucky enough to get the stats for a ranger (and we were totally fine with it). We knew we had something that worked, and we'd all seen what happens with tricked-out, class-inflated, player-catering D&D
and weren't interested. It is such blarney when people say
"I won't have fun unless I get the character I want." I think you really don't want to be adventuring with that type of player anyway.
OK. Circling back to
"Why can't a magic-user or fighter be a thief?" :
The holistic argument (above) is
"why are you trying to buck the system?" Just play a thief as a scout and it can work just fine. At low-level they will die a bit---just be cool with that, it's part of the fun.
("If at first you don't succeed...") Also, to be honest,
The Hobbit.
The "technical" argument---that will irritate those who really just want something more "combat effective"---is that in the 1e DMG Gygax (the Great Satan?) suggests you admonish a PC fighter who doesn't wear armor and avoids fighting with an XP penalty. He's playing against class...fighting the system, as it were, and doesn't go up a level. Rules like that shouldn't really be necessary, but some people can be obstinate jerks.
Perhaps this is why that rule (and Gygax) is hated/ignored, along with other anti-optimization safe-guards (like race level-limits), is because it/he is trying to course-correct against all the pressure from the player-side to break the sustainable rhythm of the game by designing
infallible PC super-men---a trend exacerbated with the release of 1e PHB and it's
candy-classes.
(There! Now I've said it. I'll show myself to the door.)