I'm on my phone right now, so screenshots will come later, but I've thought of a couple reasons having a portrait mode could be useful, particularly large tablets and gameplay recording (G&B VVS PSA, anybody?).
Some parts would be easy/ier to implement (Homescreen could "mostly" be "rotated" 90°), but the battle screen would need to be tweaked a bit, as having one side be the fight screen and the other be the card screen would be a bit awkward. Perhaps with the extra width, having the 10+1+1 cards all fit on one row?
I'll have some edits done soon as to what I'm envisioning.