NPC design is very different in the new edition. With the new design philosophy, all first level creatures (including NPCs and even PCs) should be roughly equal in power, and NPCs do not have classes. Of course a level 1 barbarian is probably more powerful than a level 1 sorcerer in most combat situations, but that's the idea. Based on this, I don't think your level 1 commoner has roughly the same combative power as a level 1 fighter or wizard, and I don't think commoner as a class needs to exist. If you haven't seen it I would recommend looking at the Monster Creation Guide. You can use this same guide to make NPCs. A commoner could be a level -1 creature with these rules.
No offense but that can't work for every NPC. The King has to be more than a level one character but there is no reason from him to be a class and power level equal to a PC. thus what this guy is proposing is not a bad idea. Sure most NPC's in town may just be level one knockoffs but not all.
Personally I don't really like the idea of advancement or whatever for most monsters that are humanoid in design or intelligent. I like the idea of leveling them up via class ability because it allows me as a DM to better gauge power levels. I just cannot see it working.
I have expressed my displeasure with this from the Bestiary where creatures like say the drow are just listed with various hit points and powers. It is too cookie cutter and takes from creativity to make it simple. It sucks and for myself I will find a way to add levels the way you do in 1e because that makes more sense. I like the idea that a 20th level party can meet a group of goblins which are their equals.
Thus exploring another ways to do this with house rules works for me better.