The Monk is a bit of an odd duck from a non-combat perspective, specifically with respect to the common challenges faced in a pulp D&D game and the niche ability packages expected to facilitate success in those challenges.
He primarily wants to be a Rogue stand-in for the infiltration shtick. However, while he possesses focus and breadth in the Stealth/Perception angle, he doesn't come standard with the Thievery/Bluff/Disguise package (while the Rogue does) that promotes success in such challenges. He can't get the group past glyphed/trapped/locked doors/chests; one of the mandatory components of this niche role in a pulp "dungeon" situation/challenge.
He can do the Athletics/Acrobatics (climbing, jumping, swimming, balancing, etc)/Endurance stuff without a problem. Put a mountain in front of him, he can climb it. Need to run all day? No problem. However, when it comes to doing the stuff to facilitate actual group success, he can't bring the breadth and niche functionality of a Ranger or Druid. He can't track, ward campsites, read trail signs, effectively camo a group through hostile, wilderness territory, gather reconnaissance through proxies (animals, nature itself, etc).
He certainly isn't going to be the lynchpin for success in social challenges or even be a momentary "Face" stand-in.
So. He can kind-of be a scout and kind-of be an infiltrator but with severe holes in his game with respect to facilitating group success. He can communicate with anyone (which is useful in the very off-chance that relevant languages aren't covered or an esoteric language is made relevant via specific, GM-engineered adversity with respect to that narrow skill)...but he doesn't have the social skills required to manage a diplomatic effort/parlay with an alien creature/culture that might speak a language esoteric enough that no other PC speaks it. He is, however, self-sustainable such that if you put a physical impediment in his way he (himself) can overcome it through athleticism or endurance.
I've never found that non-combat package nearly enough to warrant a 2nd tier combat position. I'm strongly in the "combat parity (but different stylistically) with the Fighter/Barbarian" camp. I look at the Monk and I see a mystic Swashbuckler with fists/quarterstaff/throwing stars instead of a rapier and wire-work Kung Fu instead of Errol Flynn fancy footwork.