To wrap things up for now: It's a bit tricky to say, "Human perfection has to be multiform," because what human perfection is is entirely up to the will of God.
Still, if we keep this contingency in the back of our minds, I think we can say that in order for humanity to be what God evidently intends humanity to be, individual humans must be both perfect and distinct. They must be perfect, or else God's will would not be effective; they must be distinct, or else they cannot constitute a community.
But why must they constitute a community? Or rather -- since the answer to that is "God wills it" -- why does God desire humanity to constitute a community?
Again, saying "God desires something because..." is tricky, and can only be done with the understanding that it doesn't impose an absolute necessity on God. But let me suggest two consequences of perfected humanity constituting a community, which God may (for further reasons of His own) desire.
First, the Trinity is a community of persons, so any participation in the life of the Trinity is a participation in a community of persons. Divine life is fullness, so two humans cannot both participate in the Divine life without participating in each other's life. If, then, we are not to be annihilated as individual persons, we must constitute a community if we are to participate in the Divine life.
(Annihilation can happen in at least two ways. We could be made identical, differing only in number like so many sanctified toothpicks; this, as I suggested earlier, makes us unlovable to any but a miser. Or we could somehow fuse into a single person distributed across all our bodies; we'd be the Body of Christ literally infused with a single personal soul, which would necessarily be Jesus', which would mean we would cease to exist as such.)
Another consequence of the eschatological human community is what might be called a richness of expression. As St. Thomas puts it:
[God] brought things into being in order that His goodness might be communicated to creatures, and be represented by them; and because His goodness could not be adequately represented by one creature alone, He produced many and diverse creatures, that what was wanting to one in the representation of the divine goodness might be supplied by another. For goodness, which in God is simple and uniform, in creatures is manifold and divided and hence the whole universe together participates the divine goodness more perfectly, and represents it better than any single creature whatever.
What's true of the whole universe together is in a particular way true of mankind. We are all created in God's image and likeness, but God's image and likeness cannot be expressed by creatures, not even by human creatures, in a simple and uniform way. Our differences in perfection, then, have value in themselves, apart from their usefulness in making communion possible. And what better means of communion is there than varied participations in the divine goodness?
One final point: God's perfection is perfectly perfect. The participation in the Divine life by the human community neither adds to nor subtracts from that perfection. Nor would participation by a collection of glorified toothpicks, or by a single person distributed across many bodies. It's not a question of what effect an arrangement has on God, but on what is capable of the participation God desires.
This line of thought confirms the dogma of the sovereign effectiveness of God's will. No one can be absent from the community of the saved if that community is to be perfect, as it must be. It also confirms the hope of salvation (not the presumption!) of everyone we love, whose absence would mean imperfection in the community of the saved.