I think that amongst many, Dex builds are regarded as better then Strength builds unless you want to use a two-handed weapon.
Ironically, the rapier actually needs a stronger person to utilise effectively than a Longsword.
Rapiers were paired with the main-gauche, but it was rare that they would be used in simultaneous attacks as per D&D's dual-wielding mechanic. The main-gauche was for defence and for when your opponent was too close for you to employ your rapier effectively. Generally you would be trying to keep at rapier-fighting distance, not dagger-range.
I think part of the issue is not the anachronism of them both being around at the same time, its the practicality of trying to use a rapier of said full plate.
Of course all this is rather muddied by the issue that there wasn't really a point at which rapiers just appeared. The longsword was always both a cutting and thrusting weapon, and there was a gradual evolution to accentuate the thrust, with cut-and-thrust swords for civilian use growing longer and thinner to improve their effectiveness against an unarmoured opponent using a similar weapon. The point at which these swords became the D&D rapier isn't definite.
Sort of. The rapier was a civilian weapon. For military use people would still be using what D&D would term a longsword as the backup to their polearm. Rapiers didn't see military use until quite a bit later, after armour was almost fully phased out.
I used to hate rapiers, but then I started calling it an arming sword and it all made sense.
I hate Rapiers, but I like Estocs. Go figure.
lowkey13 said:Why not just have a single, finesse (use either Dex or Str) one-handed d8 weapon and call it Awesome Sauce? Well, except it's already called the rapier.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.