Quick Key: Tankier, Offensier, Tied.
FFIII: THF - STR 63, AGI 99, VIT 63; NIN - STR 71, AGI 85, VIT 52. Tankier? THF. Offensier? NIN.
FFV: THF -STR +1, AGI +16, STA +2, MAG -6; NIN -STR +15, AGI +14, STA +3, MAG -10. Tankier? Tied. Offensier? NIN.
FFVI: Locke -STR 37, SPD 40, STA 31, MAG 28, ATK 14, DEF 46, EVA 15%, MDEF 23, MEVA 2%, HP Gain 48; Shadow - STR 39, SPD 38, STA 30, MAG 33, ATK 23, DEF 47, EVA 28%, MDEF 25, MEVA 9%, HP Gain 51. Tankier? Shadow (Gear: Locke). Offensier? Shadow.
FFXI (@75): THF - STR 56, DEX 70, AGI 65, VIT 56, INT 61, MND 44, CHR 44, EVA A+; NIN - STR 61, DEX 65, AGI 65, VIT 61, INT 56, MND 44, CHR 48, EVA A-. Tankier? Tied (Moveset: NIN). Offensier? Tied (Moveset: THF).
FFT: THF - SPD 9, HP Medium, ATK Medium, MAG Low, EVA 25%; NIN - SPD 10, HP Low, ATK High, MAG Medium, EVA 30%. Tankier? THF. Offensier? NIN.
FFTA: THF - HP 5.8, ATK 7.3, DEF 8.4, MAG 6.8, RES7.6, SPD 1.9; NIN -HP 5.8, ATK 8.4, DEF 7.2, MAG 8.1, RES7.6, SPD 2.1. Tankier? THF. Offensier? NIN.
FFTA2: THF - HP D, ATK E, DEF E, MAG E, RES F; NIN - HP E, ATK C, DEF F, MAG E. RES E. Tankier? THF. Offensier? NIN.
BDFF: THF - HP C, STR C, INT E, DEX S, VIT C, MND E, AGI S; NIN - HP C, STR B, INT C, DEX A, VIT D, MND C, AGI S. Tankier? THF (Moveset: NIN). Offensier? NIN (Moveset: THF).
Based on the above we can see that using stats alone THF has been sturdier in most occurrences and NIN has been stronger. The only FF's in which NIN is a direct improvement to THF in all areas are FFI and FFVI. The times where NIN beats out THF as a tank via movesets number 2. In one case their stats are essentially tied for tanking and the other THF wins in the stats alone. In both instances it is possible to sub skills from the other job leading both to tank effectively. In the one instance where the ninja (Shadow) outclasses the thief (Locke) in both areas the thief can pull ahead in tanking via gear possibilities.
Since THF has historically had abilities called "Mug," "Bully," "Vigilance," "Accomplice," "Collaborator," "Counter," "Steal Limelight," and "Shake Down," we could derive the theory that in a group they have accomplices whom they collaborate with to bully, mug, and shake down their targets. This results in them stealing the limelight (getting attention) in battle but that's no worry for them as they are vigilant and can dexterously evade and counter the attacks with their own.
Also as far as thievery goes in the real world, being up close and personal with a group is an often occurrence. It's generally only when the thief is alone will they worry about being sneaky and sly escapists. When their friends are with them they'll often take what they want straight on. In the game we can apply this to playing solo in the class or without the stance to sneak around with stealth abilities and escaping danger with flee and then playing in a party where you can gang up on the enemy.
Unless you show *proof* that you work for SE's dev team you can say *anything* you wish to and it will *only* be *speculation* just like the rest of us. You *speculate* time and time and time again that THF and NIN will not both be jobs.
They could potentially split Ninja into both via stance. STR for up close melee attacks and use a stance for DEX and INT based ranged attacks. Throw is a staple ability for Ninja, would love to see it utilized along with their water and lightning spells.