from my experience and testing from many korean mmo's that ive played (and played with really high lvls) the way the game stats are adjusted is by this way.
It takes +3 str to give 1'1 dmg during total 0-100 str, it takes +2 str to give 1'1 dmg from 100-200 str, and +1 str from 300+ total str... I forgot my calculations from the other games so i might be wrong and it might go by 50's, where it takes +5 str to give 1'1 dmg from total 0-50 str, +4 str to give 1'1 dmg from 50-100 total str, +3str from 100-150, and so on. and this curve applies for every other stat in what they do.
but basically the reason why ppl concentrate on one or two stats is because the fact that the stats get more efficient when higher in number, and thats the way it is in almost all korean made mmo's that ive played and tested. Spreading out stats almost tends to show a very weak char. Every char has their favorite pair of stat.
Sieg- +str, +vit... the reason why str is important is because you do more mellee than any other char, your skills aren't as repeatable as others so you'll mostly be doing mellee. since you're always in the front line, and since you have that damage reduction % skill, vit... is.. simply enough... vital... :P
dain- +dex, +int... dex coolsdown your skills and adds max dmg to it, and spell spamming is exactly what dains do so of course dex is just perfect for it (who cares about minimum dmg when spells do massive dmg). Dains just really want to spam more spells more often. Int because it allows you to stay in the action longer, spamming and killing before you run out of mana is always a good thing. you dont need vit as much because you're supposed to attack from far and from harm's way. you can win with 1 hp if the enemy never even reaches you right?, so what good will vit do..
Eir- +int, + vit . healer needs a lot of hp to make use of its heals right? and int basically means hp as well to a healer because of its conversion... makes sense? Its never expected for a healer to be the most high dmging, its only expected to survive soo long that the enemy can't do anything about it and eventually die.
of course every stat is neccessary for characters, but its definately a good idea to concentrate on one or two of their necessary stat., ill explain the pvp reasons later.