As others have made mention, cha is weak because mechanically there's nothing good about it except for classes that need it.
I've just read every post leading to this one, and in almost all of them you notice something like this: "As a DM I do..." "The DM should" etc. That's the problem. Every other stat requires no dm involvement to be useful.
You want to able to carry anything? Better have some str. You want a decent AC? You want to actually have hit points? You want skills? You want to make sure your will save isn't in the gutter? A dm doesn't have to do a thing with those stats.
Now Cha is very useful for many monsters as their spell-like abilities are all tied to cha (it is the magic stat if you will).
I have often liked the idea of cha as luck. One simply house rule I used for a while was on any roll that you rolled a 10, you got to add your cha mod (positive or negative). So it became the luck stat that could sway those average rolls one way or another.
For any of you making house rules, keep something in mind. A truly good addition doesn't just reward a high cha, it penalizes a low one. I see a lot of mechanics that give great rewards for high cha, but if your not going for the really high cha don't bother getting it at all.