Ideally, this would be used in decks that don't need shields or that have shields that wear off.
Much worse than this:

Although I assume it affects enchanted shields, the cons outweigh the pros.
Not necessarily. Consider using it to thwart Dim. Shield or Wings.
Deflag gives 1 turn to deal damage for 1 card and 3

.
This spell gives 3 turns of damage for the same cost.
Similarly, this will counter bone-shield. Deflag is pretty useless there.
Then there are immaterial shields (which you already noted)
Its basically a counter vs decks that rely on spamming shields, or using shields resistant to destruction.
As a side note, you should probably put the 3 round duration should be put into the card text since there is enough space to do so.