It is critical to understand that a full-table scan is a symptom of a possible sub-optimal SQL plan.
While not all full scans are bad to performance,
full table scans are a symptom of other common tuning problems like missing indexes and sub-optimal schema statistics (dbms_stats). For small tables, a full-table scan is better than a full-scan.
However, for a large-table, a full-table scan should always be examined as a potential problem.
Full scan I/O costs less than index I/O
When Oracle reads a table front to back during a full-table scan, we save a great deal of disk latency. Remember, 96% of the disk latency is the time required for the read-write head to move itself under the proper cylinder.
Once there, the read-write head sits idle and the platter spins beneath it, and Oracle can read-in data blocks as fast as the disk platter can spin, a processed called the db file scattered read.