I think the concept of the Borg Queen was a flawed one that started the Borg down the path of "villain decay." And the retconning of her being there in BOBW was kind of awkward.
She seems to be both embodying the collective and ruling it, but why would a collective want to make themselves MORE vulnerable by "putting all their eggs in one basket" with a personal queen? A decentralized and impersonal collective makes more sense from a strategic perspective.
As to their "nobility," if their goal was really improvement rather than conquest why not simply OFFER assimilation rather than force it on unwilling species or individuals?