Well, When I started playing FFXI it was definately NOT because of the subjob. Hell, I haven't beaten but one FF Game -ever-. And that was FF3 on the SNES (I don't care what it was in Japan, Druw = American)
I joined FFXI because it was based on PvE, not PvP, which I pretty much don't enjoy. I've done ballista once, and never went back. No big thrill or enjoyment there for me.
So I got into the game, and learned about Subjobs. I remember dreaming up these really shitty subjob combinations when I first learned about them. You know, awesome things like DRG/BST and having two Pets. lol. I find it amusing I thought of such..
Subjobs really do add to the game. They can make things interesting, they allow for customization, and can often completely change the role of a character in a party. While I like to believe thinking outside of the box is good, I remember seeing someone say something that made alot of sense: Cookie cutters are that way for a reason, they're good.
Subjobs allow you to do many a variety of things. I'll use DRG as an example.
I can sub /SAM and gain TP Faster, and almost always have TP before the other individual does. I would get Meditate, and be ready to SC almost at a moments notice. WHich is pretty attractive, if I don't care about my DMG, or I needed the TP Boost. AttacK Wyvern
I can sub /WAR and use Berserk. I get a nice power up ability that allows me to hit harder. I get Warcry later on and that helps as well. Not to mention Double Attack can help with TP when it goes off, which really isn't at that bad a rate.Attack Wyvern
I can sub /DRK and get Last Resort, Souleater, and a lil MP to use for healing breath in case of an emergency, while doing the Damage breath as well. It creates a Dual wyvern. Hell, If I a RDM is good enough to toss a random refresh my way inbetween fights, I could Poison pull.
/BLM means I could toss out some Elemental Enfeebles. I could put spikes up, if I so randomly choose, incase I do pull hate for some reason. I could cast poison in am emergency and heal the tank with the power of a Cure III, for like, 5 MP. Course he'd have to be at 33% Health (50% with AF Helm)
/WHM I could toss out buffs, a quick Divine Seal Curaga when the WHM needs to rest. I could put Stoneskin or Blink on if I was the target of an SATA. WHich would prevent DMG long enough for me to hit Super Jump. Or I could tank for a meesly few seconds, until the PLD got Voke back. I could also use healing breath on the tank as with BLM.
/RDM WHM & BLM kinda slimped together I suppose. I get Fast Cast, and spells that are basically instant, and for 6 MP. Barstone, Barero etc.
So Subjobs are a blessing. Hell, they're my bread and butter and define alot of my role in a party. Without them, I'd be massively gimped.