Nima,
Right, not all objects should be searchable, but I think it is useful to query the acs_objects table for additional attributes for objects that are searchable. On HEAD there are more object_types that have been search enabled.
On indexing additional formats, that is also on my list of things todo. The easiest thing to do is register a command to extract text from various mime types. There are utilities to do this for most common formats.
