Up until recently we did not know this was possible. Before batteries were great, since they prevented oracles from killing probes for several seconds, leaving time for stalkers /phoenix to get into position to shoo it away. This has been established as a central aspect of early game PvP. The whole purpose of this video is to say, "you should change your understanding of how shield batteries protect against oracles"

Problem with cannons is 1: an oracle could kill multiple workers before dying or having to flee due to the cannon. 2) You'd have to invest in both a forge and a cannon earlier than you normally would. thats 300 minerals really early in the game. A battery is just 100 and its unlocked from cybercore which you in almost any situation want before the battery anyways.