Well, we shouldn't. Not having the energy to change out of your pyjamas isn't an excuse to skip a day of writing, neither is not having a voice or coughing and sneezing inceasently. It's just one of those things.
A sick day from a day job is a prime opportunity to spend the day in bed with your laptop and not feel even remotely bad about the other responsibilities you might be neglecting. Of course, there's always the issue of fuzzy-brain, when stuffy-headedness, nausea or cough syrup-induced intoxication makes it really hard to concentrate on anything and all your ideas come out a little weird, but hey, that can be a good thing.
It's sort of like having a drink or two to get the juices flowing, but without the drink. The part of your brain that asks, "is this idea salable?" and "what's my target demographic?" shuts down and all your left with is pure creativity. Or pure insanity, depending on how you look at it.
I've been sick for days now and I've been using my down time - the parts of it when I haven't been sleeping or watching How I Met Your Mother to work out the plots and character sketches for my NaNo '11 story. My MC is a rabbit-woman-doll-cyborg with amnesia - and she's not even the weirdest thing about this story. I think this is going to be one of those books that I write mostly while sleeping. Wee!
So, sick days - good or bad for your writing process?