Should table and index be on the same disk??

I have the following excepts both from Oracle seemingly in direct conflict with each other. Could someone please shed some light on it???

From on-line Oracle 7 Server Documentation/Server Tuning/Tuning IO/How to solve IO problems/Separating Tables and Indexes:

It is not necessary to separate a frequently used table from its index. During the course of a transaction, the index is read first, and then the table is read. Because these I/Os occur sequentially, the table and index can be stored on the same disk without contention.

From Advanced Oracle Tuning and Admin by Oracle Press, answer to test question at back of book:

Data and indexes should be placed on different physical disks when possible. This is because typical Oracle access to an indexed table requires one or more read of the index file to determine the location of the data block containing the row to be accessed, follwed by a read of the appropriate data file block. If both of these files are on the same disk, the disk head must move from one physical area to another, and this is a relatively slow operation. I f the files are on different disks this head contention willl be reduced or eliminated. Further efficiencies may be gained by spreading both index and data tablespaces over multiple disks... Received on Fri Jun 05 1998 - 21:49:33 CDT