Creating Bitmap Indexes on a Materialized View
The key phrase is &quot;after batch row loading&quot;.  If your process does not meet this criteria then you do not need to drop/rebuild the bitmap indexes.
Jonathan Lewis has an excellent article on this.  A key point he gives is

You can get lucky -- but in general you should start with the
assumption that even a serialized batch update will be most effective
if you drop the bitmap  indexes before the batch and rebuild them
afterwards.

The whole article is good and ends with some key facts to remember about bitmap indexes:


If a B*tree index is not an efficient mechanism for accessing data, it is  unlikely to become more efficient simply because you convert it to a bitmap  index.

Bitmap indexes can usually be built quickly, and tend to be surprisingly  small.

The size of the bitmap index varies dramatically with the distribution of  the data.

Bitmap indexes are typically useful only for queries that can use several  such indexes at once.

Updates to bitmapped columns, and general insertion/deletion of data can  cause serious lock contention.

Updates to bitmapped columns, and general insertion/deletion of data can  degrade the quality of the indexes quite dramatically.