I personally think a mode outside of the normal story can add a lot of time on. An example would be spec ops on Modern Warfare 2, Challenge Tower on Mortal Kombat, Road to the Show on MLB 11: The Show, the ability to create your own levels in LittleBigPlanet, Challenge missions in Starcraft 2.
I feel these add a LOT of value towards the games. In Mortal Kombat, the Challenge Tower has provided me with a LOT of extra hours of content, and I'm still not done with it. In MLB 11: The Show, rather than the generic franchise that always seems to get old, I feel Road to the Show just provides an additional option that helps provide more value to the game. In Modern Warfare 2, I took a long time in an attempt to get 3 stars completed on each mission (I never was able to do it), etc.
It is this bonus stuff that often provides me with several hours more time into a game than I usually would have ever put into it, and thus have a feeling that the game has more value to it.
Money can't buy happiness. Just video games, which make me happy.
