Yes, three slits.
Sure, I look like an idiot but it is the lightest rain protection.
But I would look even more stupid, like a whale condom, without the slits!
My arms will get wet, and the sleeves of the shirt.
If you walk with a binliner protection, you only need a shirt on you, even in temperatures down to about +5 or so, as you otherwise get really sweaty.
I am not worried about getting my backpack wet, as I always have a binliner inside, like an inner water proof layer anyway.
A couple of extra binliners in the bottom of the pack.
A few times I have even used the binliners as a waterproof outer layer over my sleeping bag. Lower half with an intact one and the slitted one, with head sticking out, on top half.
Did Rambo use binliners too? Which movie, must see!