^Yeah it's easier. I think almost a lot of classes could fit other roles though. I mean, a lot of it is just that MMOs for whatever reason think a tank has to be heavily armoured. You could have mage tanks though that mitigate damage through debuffs and self-shields, or healer tanks that mitigate damage through heals and buffs.
I guess it just depends on how each Job is perceived. I mean, the SAM can actually do a lot in FF Tactics. SAM is one of the best healing/buff units in the game, really. I mean, it's no Calculator but until that point, a SAM using Draw is pretty good at being a healer and buffer. SAM also has Blade Grasp. You could make a case for the SAM to be a tank that basically tanks through parries and self-buffs/enemy debuffs. But at the same time, so many people still think Two Hands first when they think FFT, or Chain-starter DPS if they think FF XI.
Maybe NIN is even better example. Two Swords in FFT makes a lot of people think melee DPS but whether it was Shadow's Throw or even Ninja's in FFT as NPCs or before you really start Two Swords-ing Great Swords, using Throw and even magic, you got a case for ranged DPS, too. And, of course, Tank a la FF XI. So, do you go with what fans most associate with FF Ninjas or go with what the game needs.
Personally, I think I'd have went with SAM first for another FF XIV Tank. And then NIN as a Tank, too, if we really needed one; if not then both NIN and DRK as DPS. I think I'd take DRK as Tank last because I think SAMs and NINs were at least sometimes more often used as Tanks in the past. But DRK? FFT DRKs were pretty DPS focused with their Darkness sword attacks, FF X-2 was pretty DPS focused with all the sacrifice-HP or sacrifice-life DPS abilities, and obviously FF XI, too.
All that said, I don't really mind either way. FF XIV is totally free to try new things. I mean, before FF XI, people probably would have balked at NIN tanks... but it turned out pretty cool. So I'm all for new Job roles, too... I didn't write the above reply because I felt it had to be that way; just having fun thinking about how things could work