Personality in Oblivion is based on one gimmick: I don't trust you enough to tell you. Occasionally an NPC won't reveal something to you unless you bribe them or play that pointless personality game. You barely ever use speechcraft and it's not worth the time to train it - just man up and be glad it only happens a few times in the game.
Otherwise, there's no real difference between characters.
The same goes for mercantilism, by the time that the skill is trained enough to be useful, you'll never need it anyway. All you manage to do is gimp yourself by not picking a combat oriented skill.
If you'll excuse me, i'm going to go reminisce about how Morrowind actually had a personality system. It sucked, but at least you could taunt people into attacking you.