Books on line says:

QUOTE: Unlike DBCC DBREINDEX (or the index building operation in general), DBCC INDEXDEFRAG is an online operation. It does not hold locks long term and thus will not block running queries or updates. A relatively unfragmented index can be defragmented faster than a new index can be built because the time to defragment is related to the amount of fragmentation. A very fragmented index might take considerably longer to defragment than to rebuild. In addition, the defragmentation is always fully logged, regardless of the database recovery model setting (see ALTER DATABASE). The defragmentation of a very fragmented index can generate more log than even a fully logged index creation. The defragmentation, however, is performed as a series of short transactions and thus does not require a large log if log backups are taken frequently or if the recovery model setting is SIMPLE.

Seems to be the same cost, if not more. However, as I said earlier, switching to bulk_log mode does not save a lot of logging anyway.

Better the devil you know..