And in Steam/50.09 version --
A Dining Hall is a 'Zone' (other examples of Zones are meeting halls, bedrooms, pastures, and tombs, etc).
A Tavern is a 'Location' (other examples of Locations are temples, guildhalls, etc.).
You can only create a Location by creating a Zone first, and then adding a Location to it (with Steam graphics, using the little shield-shaped button in the Zone's info window). So you can create a Dining Hall Zone, and then add a Tavern Location to it. Since you must have a Zone in order to create a Location, there's no issue with 'overlapping' them, value-wise - they are effectively the same place.
The benefit of creating a Tavern from a Dining Hall Zone is like said above, dwarves will eat there (otherwise, with a Tavern created from just a Meeting Hall Zone, they will only drink at the Tavern - well, and dance (if there's room) and sing and tell stories and such).
What does cause value drops is overlapping Locations - e.g. having a Meeting Hall sharing a footprint (floor tiles) with a Dining Hall.
If you want to add additional 'facilities' to your Tavern/Inn - you can link multiple Zones to the same Location. So you can create a Dining Hall zone and link your Tavern to that. Then you can create some bedroom Zones and link them to the Tavern location as well, and voila - your Tavern has bedrooms for 'rent'! (Though at least right now, regular residents of your fort will still claim those rooms, rather than leaving them for visitors to use, haha).