"On the surface, it appears that necro and sorc abilities are the ones most commonly found in items."
As far is I know, this is where +skill can be found (excluding +all, +class, +type, or charged items)
Amazon: Not at all (I haven't played many amazons, though. I'm not dead sure that they cannot get skills from class uniques.)
Assassin: Not at all
Necromancer: Wands, Class uniques
Barbarian: Class uniques
Paladin: Scepters
Sorceress: Staves, Class uniques
Druid: Class uniques.
This gives Sorc and Necro twice as high chance for finding +skill than, let's say, barbarian.