1. Check the other Elemental Staves topic (that was in the first 5 topics when you posted this one...) http://ffxi.allakhazam.com/db/jobs.html?fjob=17;mid=1230931690311787944;num=7;page=1
Im currently 64 COR and it just seems that between Mkris and Joy Toy, the staffs are only going to be very situational. As meleeing for TP seems to be the best in overall cost and damage. I can see the use for staffs in obvious QD damage and accuracy (no resists), but the TP loss for swapping them in all the time seems a bit retarded.
a) Outside of exp/merit most COR will function primarily as a buffer + DD, and will often do most of their DD through Quick Draw. If you're only using COR as a non-pulling merit-whore, then yeah just stick to a multihit melee weapon and Slug Shots. There's far more to the job than that role though.
b) In situations where staves are useful, you will often just keep one on instead of constantly swapping. Fire is the prime example - if you're ever shooting for TP (say, a 51-69 level party) and you're good on R.Acc, the R.Atk bonus makes Fire Staff worth using as your full time weapon. /RNG with sushi, for instance, you will probably have good accuracy so you can use the slot to boost your R.Atk. You don't swap it out so you don't lose TP, just keep it on and enjoy the increased R.Atk and enhanced Fire Shots.
c) Pulling is another common example of a situation where you might use a staff. You won't have time to do much meleeing if you're in a fast party where you need to keep up rolls and pull. You might want a Light Staff if you're sleep staging mobs, a Wind Staff for the Eva and AGI (boosts all QD, which you will have time to fire off since they're so quick), or an Earth Staff for the Def boost and enhanced Earth Shots for damage.