“In my opinion, the dice are there for everything that can’t be determined at a table – because we’re sitting at a table. Yes, I can do the splits, and I can do a back flip, but I’m not going to do that at the table to show that my character’s acrobatic stunt should be successful (well, not again anyway). That’s what dice are for. What I can do at the table is role-play, and I don’t need dice or stat scores for that.” – Charisma Keller, Stuffer Shack
In the beginning, about 1975, man didn’t know about rock’n’roll shows, and all that jive.
What? Oh, excuse me. I zoned out there for a moment.
What I meant to say was, for a brief period in their infancy, roleplaying games didn’t have interpersonal skills – Diplomacy, Streetwise, Bluff, things like that. If you wanted to persuade the ogres that they should give you all their gold to reduce their risk of heavy metal poisoning (don’t knock it, sometimes it worked, generally the first time each Dungeon Master heard it), you as the player made the case to the DM, and he adjudicated whether it succeeded or not.
Then, starting with EPT and Traveller, skills like that started to appear on character sheets. Some argue that this allows players who aren’t that good at persuasion to run characters who are; some argue it’s just a crutch. When acting as Game Master, I get players to roll for persuasion attempts, but apply heavy modifiers depending on how well the argument should appeal to the NPCs, and often I’ll rule that the argument is good enough that the NPC doesn’t need to roll for it.
However, in solo or same-side play, these skills come into their own. When I am both player and GM, I need some way of determining an NPC’s reaction which is both fair and occasionally surprising, otherwise I might as well be writing a novel. Interpersonal skills and reaction rolls give me that.
The same applies in Play By eMail; there are times when it’s better just to roll against a skill or attribute, and move on.
Halfway Station 
I agree with you completely. However, what gets under my skin is that I find myself using Charisma and and Charisma-based skills as a crutch, when I know that I could put more into it.
Yes, it always feels sort of like cheating, doesn’t it? Mind you, before interpersonal skills, I often used to handle these things by reaction tests to see how the NPCs felt about an offer – lack of creativity on my part I suppose.
Starting with 2300AD, GDW games introduced a system where each NPC had randomly-determined motivations, which the GM used to adjust the difficulty level of interpersonal skill checks – less complex than it sounds. This meant players who did research on an NPC could tailor their approach to match his or her motivations, adjusting the chance of success in their favour. That was a halfway house that worked well for me.
@ Andyslack – THAT sounds like a cool, useful idea.