^ as weasel says..
LUK is generally inferior to STR/DEX for boosting damage. In those screenshots above, sure, there is 10 more ATK with the LUK, but the 10 str is +5% off the ATK of a weapon (if its, say, Hurricane Fury, its 5% of 332, which is 16 ATK). In general the STR score is better for boosting damage, as you get added carrying capacity and more damage, and DEX is better for boosting HIT, because it also grants cast speed (generally way more important than the HIT). LUK is good when you're overly invested in STR/DEX (105+ base each) and you still need a lot of ATK/HIT, then you can get both in one swoop with a cost-efficient 30 LUK in place of just 5 more DEX or STR.
as mentioned pretty much any build should have a minimum of 20-30 luk, but i think the main reason a lot dont is purely for aesthetic reasons.
I have 77 luk on my RG/GX/AB for arc angeling, I have a decent amount of luk on my sorc and will have on my future lock primarily for perfect dodge and a bit of extra matk.
Someone who wants to maximize cast speed and damage (example being a genetic) will be hard pressed to include LUK, as INT adds more damage than LUK could hope to, and LUK doesn't affect cast speed. So you're looking at 80-100 STR, 100-120 INT, 100-120 DEX, which leaves little room for LUK or AGI if you max those things out. If a genetic works LUK into their build, its either for potting or status resistance.
Edited by Jaye, 19 September 2011 - 06:22 AM.