When is a full table scan better than index scan?
When the 


table is small enough there is no practical difference
statistically, you'd return most rows anyway


The 2nd case needs qualifying


An index scan will replace an index seek if an index is covering
An index seek or scan with many rows that requires key/bookmark lookups will be expensive and a table scan could be better


Finally


An index scan and a table scan are pretty much the same for clustered indexes