Although I am not at level yet, I do hope to get there before event ends, my two cents are that it should be an open roll.
First we are not rolling on set bonuses, we are rolling on independent items which offer a variety of stats, granted there are differences of utility to different classes. To get ahead of ourselves seems illogical here. Say I dont make a lot of bosses i would be limited to only dicing for a few items, the set bonus might be completely out of reach for me. Case in point, I would love to have the spirit seed, stone bark, or shrivewood ammys. I understand the logic of implicating the end game value of the set bonus; however, to me in any giving round of dicing, that is not is what is being diced for. Do we expect people who already have two of there three items to not dice for those items if they come up? Or will they argue it is their right to get them and try to trade. This leads me to my second point . . . .
Secondly, I like the idea of making trades, kinda like when I collected baseball apples as a kid. Not everyone may get a chance to get a full set but they could still try to barter for the best piece they like, regardless of the set bonuses.
Lastly, nevermind, I won't even get into the class distribution inequality. I picked a rogue, yada, yada, all casters in lugh will have full sets by summers end and some rogue will pay two mil to complete his.

Pinter
On another note to address Elle's points on vitality, as a skills based rogue (i dont think many rogues are auto anymore) i use all stat categories. A rogue often spams skills as much as a mage or druid. Although my skills aren't based on focus, it is a damn useful stat bonus as I need tons of points in focus to avoid constantly chugging lixes which usually happens anyways. Thus my point about individual pieces. I would be happy with any item that offers any bonus as all stats are useful to me.