Hriston
Dungeon Master of Middle-earth
You're right, the longsword is a not a bad choice for a strength-based character with a sword and shield, and all versatile weapons are in the same sub-optimal boat of doing less damage when used two-handed than one-handed when used with their corresponding fighting style in addition to losing the ability to use a shield.
A longsword wielded with one hand does 6.5 points of damage on average with the dueling fighting style, while it does 6.3 average damage wielded with two hands with the gwf fighting style, if I'm figuring that out correctly. I don't think that's too significant of a difference, but the loss of 2 points of AC is. I think, however, the choice to forgo a shield is balanced against the other things you could be doing with that hand, such as grappling an opponent or casting a spell.
But I'm mostly referring to rogue proficiency with a longsword. Even for a strength based-rogue the longsword is a poor choice for a weapon because he can't sneak attack with it.
Obviously you can't use sneak attack with a longsword, but there's nothing wrong with the longsword being an option for a strength-based rogue to use in a situation where sneak attack won't work. Class proficiencies are a menu of options. They don't have to all be equally optimal in all situations.
And elves (who are supposed to be smart) handicap themselves by training a weapon (longsword) which requires strength when they are better suited (as a group) to dexterity based weapons.
Again, just because it's available to elves as a weapon proficiency doesn't force your DEX elf to use a sub-optimal weapon. It's there for all the elves that have a STR score that's higher than their DEX score, of which I imagine there would be many, especially in the ranks of any heavily-armored units of an elvish military.
That's what I mean by suboptimal.
There's a difference between a weapon being sub-optimal and a perfectly good weapon being used sub-optimally.