Sam needs to rebuild the men of letters... But what that really means is that he's looking for wizards and sorcerers... But are they a Watchers Council or combat wizards that come into play when their minions have exhausted their 3rd class resources?
Sam has two choices... He can recruit from bloodlines or power.
If he got together the most powerful wizards in America, they'd likely turn into a league of Evil within a couple weeks, but who knows what sort of spectra of recruits he'd end up with if the last six generations since the culling have been left to Woodstock, MTV and Twilight, gosh, it would take him ten years just to teach these people Latin.
I remember when Demons attacked magic School in Charmed and ate all the students too slow to runaway.
Centralization is a really bad idea when that impossibly outnumbered.
It's practially asking for another culling.