Well, they can make you faster: the same player, lets call him Bob for clarity, can attack faster with inverse attacks, since the speed he moves his mouse doesnt change, so doing an attack that requires less mouse movement is going to be faster. Does Bob, using inverse, attack faster than Dave, who isn't so tryhard and doesn't use inverted attacks? Not necesarily.
But does Bob, with his hand/mouse movement speed of X m/s, attack faster with inverted? Yes he does.
But of course, there is a limit to how fast you can attack mechanically, although very few players are so fast that inverted wont make them faster.
Now, does inverted attacks make sense? No, it does not, the organic feeling that M&B combat gives stems from normal attacks: you bring your weapon back, then swing it.
Instantly swinging looks spastic. Its one of the drawbacks of M&B that inverted is in the game, but even if it wasn't, tryhards would have a 3rd party program to do it anyway, much as they use for weapons, foliage, weather, etc.
Do you have to be a tryhard to use inverted? Pretty much. Used it for a while to see, got kinda used to it, but it feels wrong and looks so shitty. Went back to normal.
Will tryhard cryhards pounce on this post to point out how they are just used to it or prefer it or it feels better? Yes they will.
Also it is risky for agi builds since, in my experience, it was much easier to glance since your aim when using inverted is to hiltslash like mad, and since I rarely have above 18 STR, its easy to glance.