Depends.
Yes, depending on where I'm going and what the situation is. I have two sleep sacks, an old muslin one and a silk one. I've used them as sleeping bag liners and sleep sheets.
I like them if I'm in very basic accommodations (where the supplied linens might not be as clean as I'd like). They're great to use with a sleeping bag because they're easier to launder than a sleeping bag is. Sometimes they come in handy if it's really hot (too hot for a sleeping bag) and mozzies are around - you can get some protection from insects without suffering from the heat. The silk ones can add a few degrees to your sleeping bag if it gets really cold.
Unless you're a very very compact packer, they don't take up all that much room or weigh very much, especially the silk ones. Beware of the fake silk rayon knock-offs, though.