I'd never thought of it, but saw it in this month's Backpacker (who knew??). The idea of taking just a sleeping bag liner seems like it would work well in some situations, especially if you've paired it with a baselayer at least in your pack just in case. It seems like a good option for the area I frequent (I'm going out for an overnight tonight and the low is only going to be 66 F!). Sometimes during the summer a sleeping bag just seems like overkill!