So we've established that if you don't ride in the snow or any real rainfall, you don't need fenders. (Also, "slop" quite specifically describes half-melted snow, so if it doesn't snow, you don't just have minor slop, you have no slop at all. You can have muck, you can have runoff, but not slop.) Dry road grit isn't likely to stick to your tires enough to get propelled into your face, but get it wet and you have a completely different situation.

I ride full fenders on both bikes. I hadn't had fenders on any bike since the dinky little kids bike I learned on until last summer. Never again. I couldn't believe the difference in how clean and dry I was after I put them on. And in the snow, they're a godsend. There are some conditions where fenders aren't necessary, but if you have water or slush spraying you, you don't live in one. They won't keep your bike 100% clean, and I find that particularly with snow on a warm day, there's still lots of muck that gets on my pants. But the my upper body stays clean, and the bike is also much, much cleaner than it would otherwise be.