This occurred for me as well at a SCG: Open, but the Image was Vialed in and able to block my Goblin Piledriver.
Pyroblast can kill Phantasmal Image if it's not blue, whereas Red Elemental Blast cannot. But I still split them up for these reasons:
1) Cabal Therapy (very unlikely)
2) Meddling Mage (I can see decks wanting to get around Cavern of Souls by making you unable to cast your Goblin Ringleaders.)
3) Surgical Extraction (very unlikely)
There are more reasons I can't recall, but suffice to say that the reasons for splitting are rare, but may add up to help someday. Then again, I've already had my Red Elemental Blast almost cost me an important match by not being Pyroblast...
Does anyone have a good reason why Red Elemental Blast does something Pyroblast doesn't?!