How would pre-cast ruin things?

Sure you could do that, and with my set up, I'd have protection casted already, so you wouldn't interrupt me frequently, you'd miss 50% of the time to my 100 eval/anat, I'd be applying a bandage, I'd hit my greater heal/cure pot macro, and then I'd hit my explo/explo pot macro, and then cast ebolt, which all would hit at the same time and possibly kill you.