I know you lose quite a bit of health but getting to use the right move on their counter is pretty important, 120 base SpA is enough to 2KO a good number of quite bulky Pokemon if you predict right and Sub really would help prediction, but if everyone thinks that its not as useful I will take it off.
If Sub is not being used then Explosion should most certainly be the primary option for slot 4 without it Blissey, Snorlax and a load of other things just wall you and it can help get you out of a tight spot. Ice Beam and Thunderbolt are useful for a few things but most of them can be beaten down by your STAB moves or are not that common.
As for Earth Power Vs Energy Ball.. Rock/Ground has legendery coverage and EP has 20 more BP.
Heatran will still be able to hurt you with Fire Blast even if its resisted and if Rocky becomes popular then HP Grass may as well.
Other than Swampert what is hit harder by EB than by any of its other moves? Well a whole lot is hit harder by Earth Power, almost every rock resist will be the target of Earth Power and those 20 BP are going to cost you.
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