A biergarten, as many of us visited I. Germany, is a very different thing to a British pub beer garden and the tagging should be kept separate.

A biergarten is usually a standalone object, not an outside area of a pub.

As lostmike says British beer gardens are best covered using the outside seating tag or the is some usage of amenity=beer_garden.

However an existing tag which describes a real world object should not be changed to cover an entirely different object 

