I dated a guy for 7 years that was a pathological liar. It seemed like everything that he said was a lie...and I realized that most of it was due to a deep-rooted feeling of inferiority. Did it excuse the lying? Of course not! But if I could understand the "why" of it, I didn't get so angry about it.
After a while I realized that there really was no point in continuing a romantic relationship with this man...since I didn't believe most of what he told me anyway. I couldn't rely on what he was telling me to be the truth, so why bother even communicating? That's no way to live.
He has actually become one of my best friends because now I don't have any expectations where he's concerned. I know I can rely on him to be there for me if I need him to be - and without having any expectations of honesty in communications, we have a much better friendship than we ever had as a couple.
My XABF, on the other hand...lies HALF of the time. Figuring out which half was too much, so I began to just doubt everything. Again, not worth being in a relationship with someone that you can't trust. I can't rely on him to be there for me, so we don't have a friendship, either.
The way I see things is that lying goes against the whole purpose of communication. The truth may hurt, but at least it's respecting the other person enough to trust them with it.