In further testing it does not appear to triple agi/dex. What it does is that it doubles HIT or FLEE stats. The proof is in the testing. Turtle General has 45 AGI, and is level 97. That gives him 142 flee, or he requires 162 hit to perfect hit. If it was 3x agi, he would only require 252 hit, but in reality you need around 300. Double flee rating gives him 284 flee, or requires 304 HIT to perfect hit, right on target.
For comparison sake, I'll put HIT/FLEE rates for popular monsters with and without agi up, and power up for fun as well.
Yeah it works great on bosses, as Lupus said its the only reasonable way people can hit monsters like Ifrit (299 hit required). Quagmire reduces 10 dex and 10 agi per level, to a maximum of 50% of the targets agi/dex, and functions the same on bosses as on everyone else. People tend to overlook using some skills on boss monsters, like divest weapon to reduce attack power (and earthquake).
Hitting Cecil Damon is unreasonable normally... with a hit bow a sniper could do it if it were quagged. If you could find a decent way to blind her, it would be easy (combined with quag its 286 hit), but blinding without mist isn't overly easy. Maybe pushing a blind trap would do the trick.