A monk had a crapload of abilities, I agree he was not a good melee character, but he was better than a thief at melee ...
Were they? Because the thief levelled up significantly faster (overtaking the monk's HP at 1250XP and looking back only for two small 250XP windows and after the thief has levelled into the teens), had backstab, and (surprisingly importantly) could use swords, which were the best weapons in AD&D. For armour class it wouldn't be until level 6 that a monk would pull ahead of a DX 15 (minimum to qualify for a monk) rogue wearing non-magic leather armour.
If we retire our thief at level 10 (Master Thief) because that's when they get a guild our monk is still only at level 7 and making 3 attacks/2 rounds with an AC of 5. And hasn't yet had to duel to level up.
It's also surprising how important the ability to wield swords is in AD&D; the treasure tables are deliberately skewed towards swords as a way of favouring fighters - but thieves are along for the ride. Which is anothe problem for the aescetic monk with their maximum of two weapons and three other magic items.
At 10th level a 1E monk could outrun a horse and there were only a few monsters in the entire game that were faster than a 17th level monk.
The trick is, of course, getting to level 10 as a 1e monk. Winning three duels along the way. Our thief is, of course, level 13 by this point.
Finally the 1e monk could dual class.
I'm not actually sure they could. As I recall the requirements to dual class involved a minimum prime attribute in both the class they were entered and the class they were leaving - and there was no attribute that gave the monk bonus XP
They also required three 15s (Str, Dex, Wis) to qualify as a monk - and a 17 in the prime attribute of whatever they were dual classing into.
Alternatively he could take some levels in monk and then dual to magic user and have unmatched mobility and a far better AC than a magic-user.
Not so much. You couldn't actually
use the AC of the monk when dual classing until the magic user's level overtook the monk's. Gaining just three points of AC beyond the one you give up for not being able to use your DEX bonus required reaching level 6 - and that means that the wizard could only start using this AC from level 7 onwards. Before that they just got extra hit points unless they wanted to give up all earned XP
If we're talking about cheesing into a wizard and then have to play the thing, and we get absurd stats, the munchkin play of choice is
ranger. 2d8 hit points at 1st level. A further d8 at 2nd - and gets to specialise in a bow and the longsword. Then into wizard having given up only a handful of XP, and having 3d8 hit points to make it through 1st and 2nd levels - and serious sword and bow skills.
If you could start as a high level monk they were great - but they were sub-thieves for most of the 1-10 period.