I get what you are saying but most only the integer not what it represents.
A properly played PC would make the difference clear. Remember stats are supposed to be a bell curve, with an 18 about 3-4 standard deviations from the norm, maybe more, a real outlier. Goes the other way also.
A PC with 8 Str is noticeably weak compared to a 10 Str pc, obviously weaker than a 14 STR pc. An 8 WIS pc vs a 12 WIS pc, properly played, would show the difference in almost every session. A friend played a 5 Dex Paladin once, a unique clumsy experience was had. Luckily he had a high enough wisdom to realize that he was clumsy and avoid some trouble that could cause.
Greyhawk essentially had a “Forrest Gump” type NPC, a Paladin (Artur Jakarti) with maximum possible STR, 17 WIS and CHR but 5 Int. Great fun to interact with, as he could feel what was the right thing to do but not actually express it. Hard to DM though, as he was in a position of significance.
Another example, if you are the Bard in the group with your high CHR and you are the party face as DM I would turn to you first in almost every social encounter and not allow others to speak up right away. You better be ready to be the party face. If the party had Rary telepathic bond up then I would let others jump in from the get go, that’s a good gaming.
I really suggest in addition to trying MC try to play a PC with a handicap in some way.
I try to not get too involved in telling others how to "properly play their PC" or whether or not an 8 in their world is "noticably weak" or "commoner average" compared to a 10 being "normal" or "adventurewr normal" since i have seen a lot of games systems over the years run that differently for different paradigms.
i also try and avoid telling people that strength *should* be seen on some bell curve exponential vs a linear especially when the modifiers are strictly linear and the Dcs often seem to be too. hard to say "you are bell curve weak" when the weight you carry, the difficulty of the climb and the distance you jump is linearly mapped to those scores.
As for the Gm deciding who can and cannot speak up in character... well i guess that fits some groups. I tend to prefer to let the players and their characters choose when they *can* speak and focus more on my end on the NPCs and their actions and reactions.
For instance, maybe the CHA 8 half-orc simply dives right in, cutting off the GM-designated bard/face and starts an amazingly bad blunderous attempt at small talk destined to piss people off but very much in keeping with his 8 cha "rush in before thinking thru" nature.
To me me telling him"sorry, you are not allowed to talk yet by Gm ruling the bard is the one to go first. Wait until i tell you your character can do stuff."
But again, to each his own.
There are certainly, absolutely, wonderfully great things a player can do and maybe enjoy with a lower than 10 or lower than bonus stat on a character. i think its darn tootin' shucks darn wonderful when those stats and those players who like them are put together and blow the doors off that role playing stuff.
Like i said, they enjoy it, they roleplay it that way, its fun - great for them.
But i am pretty sure they could also roleplay the doors off a guy with a bonus stat too.
there is nothing magical to roleplaying about a negative bonus stat. its just another element you can work with.
But players do not have to be forced into it in order to roleplay and not doing it doesn't force them to not rolweplay or deny them the chance to roleplay and all that jazz.
I just think its not wise at all to jump to assumptions about someone's roleplaying based on whether they are now playing a MC character or a single class character or a character with high stats or a character with low stats etc etc etc and just judge their roleplaying on its own - if one really truly needs to scratch an itch to be a "judge" over someone else's roleplaying and fun.
To me, each player comes to the table with their own preferences - and where i can as Gm i scratch their itches as much as possible within the game. if that means Bob focuses more on running a pretty vanilla stoic dwarf who doesn't get into so much all that drama and roleplay (but Bob really likes character design and gameplay and combat/non-combat challenges.), Barry plays a rogue trickster with a nose for heists and puzzles and contacts and networking his own business (economy building) and Jim plays a halfling sorceress entertainer/mystic who really does a lot of the roleplaying things - I really see myself better able to Gm if i do not judge which of them is being run "properly" and instead focus on meshing together those interests within the games and events of the world they are in.
I can even let them decide who gets to talk first without me allowing it.
. And yes, there are places where my horse can't be used effectively. But, if a troll enters in a dungeon, a horse too. And yes, the horses tend to die a lot. That's why I have 4 horses and one of my (many) skills is Animal Handling, and I'm trying to forge a barding (until now, I have only strapped 3 small shields on the horses). But remember that a lance has reach. Many people forget how useful a lance is. And a warhorse has 60 feet of movement, and can only take the Dash, Disengage and Dodge actions. With a lance, I never enter in a position where my horse receives an opportunity attack, and retire hastily. 
