Use more than one attack every five minutes.
Your argument is illogical, you might as well say that since alchemists can heal with a potion, healers are completely unnecessary.
Being able to throw one bomb, or shoot one rocket every 2-30 minutes, is completely different than an AoE barrage of rockets similar to Blizzard or Volley. And entirely different from a class that can constantly throw out bombs and rockets at enemies.
Invisibility potion != Having the Stealth ability
Haste potion != Heroism
Jumper Cables != Having Ressurection
30 minute cooldown Mind control cap != Dominate Mind
You're being silly. Having an item or two (Because, c'mon, there's no point in listing so many items when 80% share a long cooldown) is nothing like having an entire spell set based on bombs and rockets.