So about maiming, striking and scouting.

[DevilsDontFly]

The only answer I have is aesthetic reasons, and maybe because of stats distribution, but mostly aesthetic reasons. I remember NIN used to share gear with BRD back in ARR and everybody used to say aiming gear looked stupid on ninjas. Dragoons are known throughout the franchise to be a heavy armor wielder, so it makes sense that they have their own gear that resembles the one of a tank so much. Same applies for MNK and NIN. Though to me it would have been better if SAM used the same gear as NIN and maybe scaled off DEX. That way the excuse could be that different types of damage (slash, blunt, piercing) uses different labeled sets (maiming, striking, scouting). But oh well.

Edit: To add to it now that I think about it, it's also probably to make people sink more time into the game. If all 4 melee dps used the same gear it would take you 1/3 of the time it usually takes to gear all of em to ilvl cap.