As others have mentioned, booking air/hotel together CAN be a good or great deal, even though instances of air/hotel costing less than air are increasingly rare.

To use the Las Vegas case, the effective hotel rate for Expedia's package option hotels is usually very low. However, you need to consider whether their picks are to your liking and that you can't do better on PL/Hotwire/other. IME, Expedia's offers tend to be fairly attractive for markets in which they have a sizable footprint, but are not nearly as compelling in other places. For example, in Beijing, almost all of their partner hotels are properties that I wouldn't dream of touching even if they were close to free.