When a definition query is set (or a different definition query is activated) the extent for the layer should be updated so that when you use the zoom to extent button that it zooms to the subset of data that is defined by the definition query. This should at least be an option on the page where the definition query is set, so that the user has the choice to "update extent based on definition query" or something like that so that the default functionality remains as-is, but the user has the ability to update the extent based on the dq if they would like to (and are willing to allow for the time it might take to determine that extent).
