If we use Exeggcute, we have to use mine (sorry Raseri). Substitute allows it to get to Harvest without risking the OHKO. Exeggcute with Substitute abuses its infinite health with Toxic Spikes to great success (it's best to pair it with Diglett in order to beat out the fucker that is Croagunk). Likewise, Substitute lets it get a heavy hit on something that would try and break its sub, but would otherwise kill it (Taillow, Gastly, switch in from Mienfoo's U-Turn, Scraggy, Doduo, Shelmet, Larvesta, Darumaka, etc etc). It just provides a lot more utility. HP Fire isn't needed at all.