redyp,

I do not know if rebuild indexes is (or was) 'advisable' anytime, but in 9i, you have the option to coalesce indexes and many DBAs prefer that over rebuild.
As per Tom Kyte and other Oracle Gurus, rebuilding indexes is a waste of time. However, I feel it depends upon the application and in an OLTP environment, with large number of deletes/ updates on the indexed column, you might want to pick indexes that require rebuild based on deleted leaves.
I have'nt had much of a chance to check what happens in our application on 9i without a rebuild over a long time (we have migrated sites to 9i in the last 6-7 months), but I know for sure that the same application performed much better after index rebuild on our oracle 8 sites. Particularly, I also noticed ORA-0600 errors while analyzing indexes in Oracle 8 when they were fragmented.