I had hoped I made it clear I don't think this is primarily a RP issue. It's a mechanical issue. Why should the player, who envisages his/her PC using a scimitar rather than a longsword, suffer a mechanical penalty? How does that make the game better?The scimitar issue is taking a mechanical abstraction (the weapon statistics,) of another mechanical abstraction (hit points,) intended for one context (quick and easy combat resolution at the expense of detail) and applying it to a completely different context (character image and concept.)
We know that scimitars, sabres, tulwars etc were used in our world. That would appear to indicate there there are factors in play that aren't covered by the rules - a fact that we're all already aware of: D&D just isn't granular enough to cover the sort of intricacies that lead to the development of different weapons, armour, fighting styles etc.
A player might take the inspiration of their character from this, perhaps envisioning their fighter having a whirling, athletic fighting style that fits well with the shape of a scimitar and that it is the traditional weapon of their culture.
To them, the 0.65 DPR damage potential difference is less important than their enjoyment in playing a character with a strong image that they like.
Would you really want to be the sort of person to accuse them of 'bad roleplaying' because the rules say that their character is proficient in longswords as well and they are playing a stupid character if they don't start using them because they would do 0.65 DPR more if they switched to longsword?
As I said, the only answer I can think of for insisting on this character's scimitar doing d6 rather than d8 is that we deny that this is all abstraction - that we think that it matters to our sense of the shared fiction of the game that longswords are heavier, more dangerous weapons than scimitars. But now we can legitimately ask, why is the character using the lighter weapon? And we can ask that in character! (Which is [MENTION=6775031]Saelorn[/MENTION]'s point.)
In my BW game, the character who choose to carry a shortsword rather than a longsword despite being equally competent with both had an answer - she was an assassin, and wanted to be able to hide her shortsword in her backpack.
Maybe the scimitar-wielding STR fighter has a similar answer (though I'm not sure that, at the same weight as a longsword, the scimitar is actually all that much smaller). In which case I'm happy to hear it - though it, as [MENTION=6775031]Saelorn[/MENTION] mooted, the answer is "tradition", one might reasonably push back against this in character.
TLDR: if it's really all abstraction, why is the player being penalised in the name of fetishing the weapon chart? if it's not abstraction and actually means something in the fiction, then the character should have some answer in the fiction.