DEX is not entirely useless in Flyff. It's highly important up until a certain point in fact. DEX adds only two things for a Power Jester: Accuracy and Critical Rating.
1. Your breakpoint lies in two skills: Haste and Gerburah Tiphreth (aka GT). Fuck breakpoints and cookie cutter builds. A Jester with 15 base DEX has 47% aspd. Add Haste/GT to this and you have 97%. Add the Cannonball buff to this, and you're at a nice 100%. Done deal.
2. If you want to do is solo, then make a battle RM or a BJ. Anti social people and kant-find-are-em people don't need a Yoyo Jester, sorry. Make some friends, find some RMs, etc. I have friends from all servers that are YJs, and they were all able to find RMs, so you can too.
3. Your accuracy, however, won't max out with 15 DEX. Depending on what you kill, your accuracy will suffer. Less accuracy means more misses, which in turn equals less damage over time. Needless to say, a Power Jester needs maximum accuracy to fully utilize its powerful attacks. The strongest hits are useless if they miss, and no amount of damage can compensate for one.
The Accuracy buff, Cannonball and a +'d set will greatly improve on your eye-sight. With a +10 set, you don't even need to worry about accuracy at all combined with RM buffs, but a +8 set might need some extra DEX to add to the hitrate. Seeing as it greatly depends on what you fight, it's difficult to tell just how much DEX you need in order to obtain maximum hitrate. Around 40-50 DEX buffed with a +8 set, however, should grant you immaculate hitrate. Once you obtain a +10 set, you can decrease your DEX pretty much as far as you want.
4. Critical rating comes in second when it comes to important DEX-related issues. Critical Swing already adds 40%, your 90 set will add 15% more. This means you won't need much DEX to achieve a comfortable critical rating. I will personally never recommend any lower than 83% critical rating. It seems that anything less just doesn't critical a lot, and thus you lose a tremendous amount of damage. I've tested 75%, 79%, 83%, 87%, 91%, 96% and 100%. 87% is my favorite, while 83% is in close second. Anything more sacrificed too much STR, and anything less sacrificed too many crits.
Note: a Jester class quadruples the DEX:crit ratio. Every 10 DEX means 4% critical ratio, rather than just 1%. I'll put up a small graphical of critical ratio.