I think a better way to implement disposition and speechcraft is this:
1. disposition of an NPC is determined by what you have done for/to the NPC, what you said to the NPC, your reputation with the NPC's community, your fame/infamy, and maybe some bribing as well.
2. speechcraft skill is increased by talking to NPC's. Sounds boring but that's how it works in real life - you practice talking. For example, every time you go through a fresh topic (like in Oblivion, the one in bright golden color) in a conversation, you receive a few XP for speechcraft.
3. to utilize speechcraft, we can let the player gain access to new topics when the combined value of speechcraft and disposition is above a threshold. And some perks too.
4. do you remember in oblivion there are a lot of (I mean, a lot of) quests that involve having to play the speech minigame and raise the disposition to a certain point before you can move on? not very creative way to implement quests, I think. There should be alternate paths to take rather than persuasion.