Vintage Leather, I appreciate your perpective. I have had some "accidents" that I think make a bag beyond use (huge ink stains, horrible mold, and a tragic mothball curse when I loaned a bag to my SIL) and would love to know if those things would make you get rid of a bag?
MOLD!

I don't know what I would do if I ever found mold on one my bags. Probably plan evil and devious things to whoever borrowed it for me. I'm a bit terrified of bad mold (long story involving bad plumbing in a converted brownstone) and actually invest in a dehumidifier when I am living in a humid climate.
Anything up to but NOT including mold can be saved. I tend to receive my families semi-dead bags, just because I enjoy bringing them back to life.
Mothball - I actually would leave it outside until it aired out. (Note, this doesn't work in December...) I want the bag to breathe, and see if it can save itself. I would also try lavender satchets. I used to swear that lavender could save everything - I am probably wrong, but...it never hurts to try.
Ink Stain - Inside or outside? If it is inside, on a cloth lining - clean it. I actually do have a solution for removing ink from fabric (Amodex) - one of the curses of writing with antique fountain pens. If the lining is leather, just accept it.
If it is on the outside, I would redye the purse.
The Noe which has become my knitting bag was a gift from my aunt. She carried it until holes had worn in the base. I took it to LV, and it would cost more to repair the bag than to buy a new one. Between its delicate state and the fact that I really needed somewhere to put my knitting - it seemed like a perfect match.
If I've never seen the problem before - I appeal the Doc, and see if she has any suggestions.