There's nothing wrong or illogical about having the Imperium consisting largely of undead, Sarellion. I made it clear in my original post on them that it was a small nation on Aleasana's border. A small number of necromancers and evil clerics can only do so much animating, and can only control so many undead at a time. Inevitably, if they don't keep a close reign on the more chaotic undead, they WILL be overthrown by anarchs, hungry ghouls who just have a sudden urge to eat the nearest living thing, enterprising wights and such who think they should rule instead, or wraiths who just want everything fleshy to die. Or rival vampires, liches, or mummies who want to take the place of the living rulers. So they are definitely going to have limits on how many undead they create, based on how many they can control or keep track of at any given time.
And after all, they need corpses to animate, and those'll be hard to come by if their opponents have anything to say about it (i.e. burning the dead on the battlefield, for instance). Most undead are not intelligent anyway, and are not very powerful, so they're usually easy to destroy if an army of them starts marching. They may be tougher than living warriors, but will have more limited tactics and strategies, slower movement (generally), and ultimately be much more dependent on their leaders surviving. Note that spells like Create Undead and Create Greater Undead are fairly high-level, so very very few casters will be capable of creating anything more than mindless zombies and skeletons. And most of the casters won't be very high-level, so won't be able to create nor control more than few undead. Then they have to go risking themselves if they intend to direct their undead forces against a military target.....
I have no problem with Odraani having some good or neutral undead, but it is likely to be overall evil or at least run by evil undead. If they actually go out seeking to "liberate" corpses and spirits from elsewhere, then it's almost certainly just because they want whatever knowledge or power those dead may have, or want to bolster the ranks of their undead army through corpse-stealing. Regardless of whether or not the undead are treated well once inside the Imperium, their overall purpose is likely to be vengeance upon Aleasana or something.