Artifacts in Heroes III.

even when not playing as Necropolis, if you have the Cloak of Invisibility, you can buy a Necromancer hero in the tavern, quickly level up his spellcasting skill to expert, and go for it!

Well, it depends. If the map has the maximum number of players, it might not be possible to hire a hero with spellcasting. I once played against 7 AI opponents, got the Cloak of Invisibility, but couldn't hire a hero. Either they were all hired, or I just had bad luck with the heroes available, but I couldn't use the cloak.
So, it's not always useful when playing as other factions.