For fast paced MMO yes, lot of instants works just fine. For XIV... I don't want gameplay to be tailored for mages who are supposed to use instant spells because of the latency issues.

If let's say we have Sleep which become instant, it means that the gameplay will take that in consideration and we may have a boss encounter where players will be given a second or so to sleep popping adds let's say. What will happen in that case? JP players? Fine, latency is low, they'll sleep. NA? Well latency is average they may be able to sleep. EU? Screwed as usual.

Same goes with any spells. So if they make it instant, then the gameplay will have to be adjusted for that, making it really difficult for those who have to bear with a high latency.

However when a spell casting time is 3 seconds, latency weight less in the average time a player will need to react + cast a spell, so everyone will more or less on par.