This is an interesting topic. I'm a born-in as well, and I think I'd agree with your feeling that there's no reason to forgive someone who's just going to keep being a terrible person. But then, I'm also an extreme introvert and I take pride in my self-sufficiency. Combining those two things means that I get very little from relationships (I don't connect well with people, and I almost never ask anyone for anything) and so I find it very easy to move on if people treat me poorly.
On the other hand, my mother-in-law (also born-in) is basically a doormat. Litterally all of her friends take advantage of her constantly, only to treat her poorly later. They're happy to ask her to host a party (and then assume that she's going to cater it as well) but then a week later they'll exclude her from something else. She continually forgives everyone only to have it happen again. She needs the social interaction, and is willing to put up with a lot to get it.
So, while it's hard for me to be 100% objective as a born-in myself, I'd say that it's probably more to do with your individual personality type than anything else. If you have a strong inner need for social interaction, or if you're sentimental and place intrinsic value on relationships based on relatively arbitrary qualifications (i.e. you'd forgive your estranged father for something that would cause you to cut ties with a mere aquaintence) then you're more likely to see a high value in forgiving people. If you're already at or above capacity on social interaction, and you have no sentimentality (like me) you place a lower value on relationships and are therefore less likely to forgive. It's all about the comprimises that strike the right balance for you personally, do you often feel lonely or socially deprived? Then you need to lower your standards a bit. If you tend to always feel exhausted from the pressures of having to interact with too many people? Then you can afford to cut off the folks that treat you the worst.
Edit:
I should probably add that I mean none of this to say that I (nor should anyone) continue to harbor ill-will toward anyone. I tend to be quick to forgive in that sense, as there's no point in dwelling on anything that you're not going to act on. Since I know I'm not going to seek revenge, why dwell on it? I'm also not talking about cutting people off entirely like the JWs do when they shun someone. But I won't seek out association with someone if there's not a high likelyhood that it'll be a positive experience.