My Empire armies usually have 2-4 artillery pieces - mortars (later replaced), great cannon or Hellstorm (somehow I don't believe in Hellblaster). This works fine, but once the enemy is close, artillery is nearly useless and vulnerable.
Steam tank on on the other hand is amazing in close combat - not only it has heavy armor but also causes fear and terror.
So is there any reason (except the cost: ~500g per tank per round) why I should keep regular artillery instead of tanks?
Edit: Since the Steam Tank doesn't show until the late game, your usual enemy at this stage is usually the Chaos invasion/Norsca with lots of heavy infantry, chariots and hellcannons