Going by the baseline, XP doesn't measure importance. It really does measure # of kobolds bashed and # of traps overcome.
So a 15th level commoner is likely to be on the edge of civilization, fighting enemy armies (malitia-style) or marauding monsters (again, using his pitchfork if nessecary).
I mean, at least one monster in the MM, the Ankheg, regularly makes a mess of farms. 15th level farmers are the kind of dudes who've beat down Ankhegs before, the kind who trolls and giants defer respect to. He can make corn grow in sand and has picked up some decent fighting prowess. Nothing military or trained, but he can swing a stick like a friggin' monk.
I've always supported the DMG's definition of commoners being low-level by default. I mea, SKR's system is all good and such, but it does lead to many PC's being not heroic at all until they get to high levels. Not nessecarily a bad thing, you just have to remember that in a town that competent, they're not gonig to need as many heroes. "Up, we've got orcs again. Time to get the boys together and go beat 'em up!" "Aw, dang! Another dire wolf eatin' me pigs! Gamps, you grab the shotgun!"
Keep in mind that NPC's also have treasure by level. Just using an NPC equipment generator, I get a 15th level commoner with the following:
Com15
Masterwork Mithral Shirt; 10 Masterwork Bolts; Heavy Crossbow +1; 1 Bolt +1; Potion of Swimming; Potion of Cure Light Wounds (x2); Quall's Feather Token (whip); Heward's Handy Haversack; Glove of Storing; Eyes of the Eagle; Universal Solvent; 146 gp
Meanwhile, even a 5th level commoner (not that rare by SKR's standards) has:
Com5
Padded Leather Armor; Masterwork Sling; 5 Masterwork Sling Bullets; Potion of Cure Light Wounds; 2 Flasks of Holy Water; Thunder Stone; 418 gp
And at the high end of the default system, the toughest farmer (at level 3) in the fields has:
Com3
Masterwork Studded Leather Armor; Morning Star; Potion of Swimming; Potion of Cure Light Wounds (x3); 17 gp
Now, as commoners, most of this is probably stashed back in their barns, just waiting for enterprising adventurers to reward.
From a mercenary perspective, adventurers would seek out the outlaying towns for the best stuff. After all, more challenges = more levels = more treasure. Of course, high level commoners = tough challenges, too, since they're not likely to call in adventurers on anything they could handle themselves.
I think if I made an edge kingdom with 15th level commoners, my PC's would just kill them and take it all. But they're evil, so. ^_^
