Why are there 5 oceans?
The Southern Ocean does have its own current, the Antarctic Circunmpolar Current. 

It helps generate the Circumpolar Deep Water due to density gradients. Ice melts and thus salinity decreases. Differences in salinity are the reason for bulk water movements in the ocean.

Thus the Southern Ocean contributes to cool the Earth. 

Geographical criteria may be open for discussion, but oceanographically speaking it is a separate system from its neighbors, the Atlantic, Pacific and Indian.