Why do prokaryotes have all those DNA polymerases (I, II, III, IV, V) if DNA pol I has all the needed activities (3'-5' exo, 5'-3' exo, 5'-3' polymerization)? Why not just have DNA pol I????

The different polymerases have different capabilities, e.g. with dealing with DNA defects (either detection or bypassing "roadblocks" due to lesions) and are required at different stages in the life of the cells.

It is not possible for a single enzyme (DNA polymerase I) to carry out the complex replication of DNA. You can refer to http://en.wikipedia.org/wiki/DNA_polymerase to have a deeper understanding of the different families of polymerase, all have unique functions of their own.