Does it have to be a day? Multiple days? I limit case work to 2 hours, on something we've already solved. Too many of these interviews apparently seek free labor and ideas.

What's really simulated by going off and building something on your own start to finish? You miss out on all the collaborative skills required to ensure you're writing the right code.

If you only care about hiring programming, motherfucker (google it) then by all means.