[Ikonoclastia]

Dexterity

This determines how hard you hit with your bows, how quickly you learn weapon and rogue skills, and how often a magic weapon will process it's skill. This is a good skill to have for fighters, especially rangers and rogues, and of only limited use for magic users.

http://everquest.allakhazam.com/Creation.htm

Dexterity: Dexterity affects how quickly you learn weapon skills, and how often weapons will do their special. Since Shadow Knights get a number of weapons which proc, and some spells which do this also, there is a definite advantage to putting some points into this statistic. It will also help to increase their channeling skill so they are not interrupted as often when hit while casting.

http://eqthieves.com/class-shadowknight-creation.htm

Both Player comments but this pretty much clinches it, its a supplement for the original EQ manual:

Dexterity:
This is your hand/eye coordination and is used in combat accuracy and spell casting capability. This is used in calculating your missile combat skills and aids in determining if you are interrupted when you are hit while casting a spell.

http://www.angelfire.com/nt/com1/playg.html

Edit: theres most likely 3 checks, agility (avoids getting hit and interrupting), dexterity (you do get hit and determines if you are interrupted), channeling (if you are interrupted whether you continue casting the spell). So they're likely not wasted though the degree's to which each helps I have no idea