I leave the hood on my Sigmalux a lot, simply because I don't want to lose it. It won't fit in the bag well if it's on the right way around. It really is just motivated by a desire not to lose the thing, though.
I think it's equally silly for people to leave their hoods on the right way in grey weather/after sunset. For every clueless dSLR owner who doesn't know what their hood does, there's another who keeps theirs on when it's unnecessary because it makes their camera look cooler! (Though as the guy above me wrote, they can be excellent for damage prevention).
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