Nitpick perhaps, but if Watson is assisting Sherlock doesn't that merely give Sherlock a small bonus on his roll rather than giving Watson a roll of his own?
It would give him advantage, except the other option is that both players attempt the roll separately, and that's what Watson is doing here, but calling it an "assist." Because everyone knows Sherlock is dumb.
Also, your point is valid if-when things are decided mostly by rolls. In a less-structured setup where things are more driven by actual roleplay (i.e. most social situations, planning and tactics, and some exploration) then the dice can't cover this off nearly so well.
Yeah, and that's where I don't like forcing players to RP to the arbitrary concepts loosely represented by attribute scores. What it can mean is players or the DM policing each other's RP choices and contributions to strategy, etc.
I run a weekly game with teenagers, 13-17, mostly novices. I think it's
wonderful when they RP even a little, and I am not going to quash their choices (well, unless they violate our session 0 agreements, but that's a different thing). My home game has different levels of experience, but IMO any RP is good RP, again as long as it is within our agreed upon boundaries. If a player
wants to play their character as kind of dumb (the atribute that mostly gets RPed) then that's awesome. But I'm not gonna enforce it (though, full disclosure, I love playing my Int. 8 monk as kinda clueless).
IRL, intelligence is not a single thing. All folks are brilliant at some things and hopeless at others. An Int. 3 character is still a functional, highly skilled person, per the rules, so obviously a 3 is not meant to represent someone incapable of being an adventurer. Instead, it represents their facility with specific D&D functions, like casting spells or (in the case of 5e) Investigation checks and the like. Maybe they're a math savant as well (c.f.
The Hangover) - that would be fine with me. I would never tell a player "No, you can't play your character that way" because of some arbitrary numbers.
The rules already take care of what happens in the cases where the attribute is meaningful in a game sense. A character with Int. 3 is not going to make a viable wizard or investigator. But they can act like they are all they want.