[The following is based on my experiences in 3.5e, but from what I know about Pathfinder it should be trivially adapted. Also, I apologize in advance for what I'm certain will be a post filled with incorrect terminology -- I've been playing 4e for quite some time now, and it's been even longer since I last sat down with 3.5e.]
If you think Diplomacy is broken at level 5, just wait until you get to the Epic levels! This is where we were when my DM decided to address it.
They way he approached it was to completely ditch the static DC list -- static DCs make sense for climbing ladders (which don't typically get harder as you get higher in level), but they don't make sense at all when you're dealing with more and more experienced and powerful individuals; just like AC and other such things, as the CR goes up so, too, must the DC.
So he sat down and took the table of Diplomacy skill DCs and turned them into situational modifiers. I think he started with "Neutral" granting a +4 (reasoning being that changing people's minds is not easy, even if they don't dislike/distrust you), and then each step toward Hostile added an additional +2, while each step toward Friendly added a -2.
The resulting modifier was then used on the NPC's own opposed Diplomacy roll. Thus the table of Diplomacy DCs that is so trivial for PCs to game was gone, replaced by opposed checks to modify a character's attitude.
But he went even further. Between each stage on the "trust continuum" (i.e. Friendly, Neutral, Hostile, etc.), the DM added a "half step"; a successful Diplomacy check would move the NPC's attitude half a step, not a full step, thus requiring 2 successes to effect a change in the character's attitude. (When an NPC is on one of these "half steps", his/her attitude is the one "rounded" toward neutral; thus an NPC is effectively Neutral across 3 distinct "steps", but 2 "steps" for all others.)
Finally, he added one more thing: Continued successful/failed checks could move an NPC further than the ends of the "attitude spectrum", although no further mechanical advantages were earned. What it did do was make it less likely for the NPC's attitude to be changed later, by simply keeping track of how many "steps" would need to be adjusted.
These were the mechanical changes he house-ruled into Diplomacy. He also required certain role-play elements to also be met before a Diplomacy check could even be attempted -- the Halfling Bard walking up to the dragon and rolling an impressive 34 Diplomacy is just wasted effort if said dragon isn't even listening! There were also common-sense limitations imposed: a dragon who's entire life is centered around accumulating his horde is not going to just give it up, no matter how many Diplomacy successes the Bard accumulates!
When a bigger hammer doesn't suffice, intelligent heroes need to use Guile and Cunning... which sometimes includes talking enemies to death.
The concept of the "face" in role playing games exists because of some problems that cannot simply be solved by beating them to death with a bigger hammer. These problems include: getting paid, finding a gig, explaining that "it wasn't us" to outraged authorities, and all sorts of other... politics.
The need for a face, for a bard, is directly proportional to how much political wrangling the party faces. If getting paid is a matter of dropping a head on a desk somewhere and saying "gimmie my money" and... they do, then there is no need for a bard. If there's a entrenched bureaucracy between your (carried) head and that desk; suddenly there is a need for a bard.
Incorporate political problems and consequent bonuses to make playing the face appealing.
Players should be able to earn greater bonuses, avoid some combats, get assistance, and get paid because of the abilities of the face. So long as the group is willing, this can also include scouting duties and other unsavoury "we don't have a big enough hammer to solve this problem" problems. However, since some people play RPGs to get away from politics, this is absolutely something that must be discussed with the group first.
Best Answer
You are probably forgetting the spell DC formula
Meaning that even though you will be able to cast your spells with low Charisma, they will be far more easily resisted (successfully saved) by your opponents. Of course there are spells which don't require a save (like all of those that are beneficial to your party members like Cure Light Wounds) so maybe low Charisma could be a choice if you don't really want to focus on enchantments and targeting foes with your spells.