In looking to book a trip, I went directly to an airline's website and found they allowed you to book multi-city trips with car rental and at least 3 days hotel (even though the trip is more than 3 days). The combined pricing was very good. The cost which included business class, was less than booking the business class tickets alone.

I have two problems, I do not plan to use the hotel stay. We have family in town and we will stay with them. The 2nd problem is, one person in our party of four may not use the return flight of the trip. There is a chance they will go onto another city to meet up with some friends and travel home from there. Despite all of what I'm not and may not use, the total combined pricing is still very good for the airfare and car rental.

I have been through the terms and conditions and I do not see anything that would prevent me from not using portions of the package (I am still paying for them). If this were through a travel site I would be very hesitant, but this is all within an airlines site.

Does anyone know any reason I should not do this?

It's unlikely you'll have any troubles on either front; people no-show for hotels all the time, and no-showing for the flight home is usually not an issue unless it becomes a recurring behavior.