In particular, how do you feel about some of the being "common" or not? I have taken a look at the different series (Harbringer, Dragoneye, the coming Archfiend...) and sometimes I got the feeling that in WotC's opinion common=low-level. But it is really what DMs need? For example, why all those human or dwarf warriors, crossbowmen, archers which seems to be almost in every package? After your campaign is mid- or high- level, do you still face this kind of NPCs? Since also the most common campaign uses good-aligned PCs, it doesn't happen often to fight against other humans or demihumans but rather against evil monsters. Of course it helps that you can simply say the human crossbowman is a 10th level ranger if you want, but at least my players don't like killing humans or dwarves or elves.
I think that what is common in the packs is based on a power distribution that they have created for the Miniatures game, not for DnD. They are not necessarily thinking about a DMs campaign when they determined the randomization but about how it would effect the miniatures game. For me, this happens to work well enough for my DnD game. I use a lot of humanoids as opponents for my PCs. These humanoid warrior and crossbowmen models are perfect for using as random mook models. For those I don't care too much about particulars, but it is nice to have something that somewhat resembles what it is supposed to be.