I think that living a virtuous life, even without a carrot or stick at the end, is worthwhile in and of itself.
Sin begets sin. If someone lies, they lie more to cover it up and so forth. If you stole once, you are more likely to do so again. Hate leads to violence over time, etc.
So by acting virtuously, you are effectively making the world a better place itself. If people acted this way, a tall order I know, then everyone would be happier. The Stoics didn't think they would be rewarded, yet tried to live well. Even the older Atheist thinkers thought you should keep to the moral precepts, a Noble Lie in their opinion, for the betterment of all. Practically, sin leads to ruin over time, of either the sinner or others that are negatively impacted by his sin. Either way, for the good of society, it should be opposed.
This even applies to things like sexual continence or avarice or gluttony, that are more controversial today. There is such a thing as excess, and a golden mean needs to be followed. For instance, a marriage where both are devoted solely to one another, would obviously be more succesful than an open one, which is only thought desirable today because people no longer value virtue for itself.