When I moved to the UK, I was initially quite wary of leaving leftovers and eggs out. But DH always did it (and reasoned that loads of people in the UK do it and they're not dropping dead left and right), so I started doing it as well. It's been fine, I've not suffered any ill health effects from it, and I don't even think of it anymore.
Obviously, it depends on what the leftover is and how long you're going to leave it, but I'm totally converted now to leaving more things out.
That's me, too. My husband leaves
everything out, and has no qualms about eating something that's been sitting out for a couple of days. I'm still quite wary of it, but leftovers are usually kept for his dinner at work the next day, and I just have a fresh sandwich since I work at home. So I don't have to eat it, and it doesn't seem to negatively affect him.
But I do start getting more insistent that he put things away when the weather gets warmer. During the winter, our house is basically one big refrigerator anyway, but if it's warm enough for butter to melt (not just soften), I make him put things in the fridge. It's the rule that has become the compromise.