I don't hold my knife like a pencil, but I don't understand why it is so upsetting for people to see others doing so. Someone somewhere decreed that there is a 'correct' way to hold a knife - I doubt anyone now knows who, where or why - and all these years later it is used to suggest that people holding it differently is offensive. Maybe they're just doing what the 'non-pencil' people do, which is hold it as we were taught as children, or maybe they just prefer to 'pencil' their knife? Why is it offensive?
I agree that shovelling in food, that showing what is in their mouth, that dangling food over your mouth (!!) is off-putting to others, but I'd rather someone held their knife differently from me than sit there scrutinising how far others conform to perceived 'standards' and judging them.
Something I do judge is people taking the best bits of a dish for themselves - the crispiest bit of a cheese topping, the best looking roast potatoes or whatever. If I've cooked and there were an obvious discrepancy between one bit of a dish and the rest I would always ask guests to take the best bit (and want them to do so), but when people swoop in and do it without being told I can't help noticing, whether I am a guest or the cook. To me, that says far more about someone than they way they hold a knife.