This PDF by Richard Foote uses quotes from Burleson and Niemec which he disagrees with and considers 'myths'.

Rebuilding The Truth

http://www.actoug.org.au/Downloads/o..._internals.pdf

The vast majority of indexes do not require rebuilding

Oracle B-tree indexes can become “unbalanced” and
need to be rebuilt is a myth

Deleted space in an index is “deadwood” and over time
requires the index to be rebuilt is a myth

If an index reaches “x” number of levels, it becomes
inefficient and requires the index to be rebuilt is a myth

If an index has a poor clustering factor, the index needs
to be rebuilt is a myth

To improve performance, indexes need to be regularly
rebuilt is a myth