Talk:Firestorm/@comment-124.171.29.107-20160807115833/@comment-4511886-20160807120308

The main reason was so that being staggered mid-cast wouldn't completely waste the spell.

Either way, it was fixed in Dark Souls II. Then they went back to it in Dark Souls III, but at least it makes more sense there due to using the MP/FP system instead of casts.