Quick Analysis: Mage changes and Final Fantasy Tradition.

[Firon]

I think FF13 was the first FF game where debuffs were worth their use in many scenarios.

Spells like Slep, Rub, Qake, Bane, Zap!, and XXXX.
Or Poison... It seemed like encounters weren't built around the party using these to stop the enemy. Versus a twofold spell like Virus that deals damage and quickly depletes life.

Yeah i can honestly say FF 13 is the only I remember casting any debuff's, oh and Slow on Seymore's(think that's right) pets in FF 10, besides those 2in other FF games they were pretty much useless.