You don't need to have ANY ranks in a skill to do OK in it.
Sure, Fighters who use Diplomacy may not be very good at it in general, but against your average Wis 10, Cha 10, Int 10 villager, they will make most of the checks about 50% of the time.
Diplomacy...Bluff...Knowledge...how many of these skills can't be used untrained, exactly?
In the same way, a wizard can wade into melee combat and potentially not do too shabby.
All ranks and BAB do is increase the likelihood of you doing well. They don't make you suddenly able to do something.
I have a psion/sorcerer who specializes in melee combat. The dude isn't nearly as effective as a fighter would be, but he's not too shabby after all the buffs...
I similarly have a fighter who goes the Expertise route. His Int of 14 means that he's pulling even with the ranger on skills, and he doesn't suffer too much in other areas (his Dex ain't that great, but I give him big bulky armor, so he don't need it.

). He's not nearly as skillful as the rogue, but that hardly means he's incompetent.
Listen, you don't need high ranks to do well. A cross-class here or there, a skill focus, or just a decent stat of Cha or Wis will allow you to do VERY well in noncombat arenas.
I mean, haggling down the price of a longsword isn't a DC 25 activitiy, y'know? Setting up a fighter school isn't DC 30+. Getting free beers at the local bar isn't DC 20.
These are DC's of 10...11...12...maybe 15 at the highest. ANY character can do well in these checks just by makin' a little roll, and maybe having a decent stat.
See, those social skills exist so that there's an actual reason to have a decent Cha.
If you walk around without one, it's not the game's fault you can't interact socially...it's yours, because you value being able to move faster or an extra hp over getting free beers.