Mercurio isnt really good, nor is J'zargo... Illia, on the other hand, is AMAZING.. she casts a lvl 2 ice spike, and may only scale to lvl 30, but when i was lvl 50-60 she was doing major damg to not only minor enemies, but bosses even... I was so sad when i lost her to the one time bug! Once shes dismissed (via quest or manually) shes unobtainable a 2nd time.. Not onyl is her spell base powerful, but she uses any, and EVERY staff you give her! Many will not use much of the staves given.. She will, and does so effectively! Illia is by far THE BEST follower ive ever had, in over 2000hrs of Skyrim!
Enchanting IS the mages craft... Blacksmithing is for heavy armored warr/barb types, and alchemy (arguably via poison) is the thief/assass craft... Enchanting is most beneficial for mages... Not to mentionthe destruction perks that add fire/frost/electri dmg, also affect their enchanted counterparts...You will need enchanting as a mage!