logically speaking it shouldnt be dynamic cost you could potentially exploit the hell out of it if it were, coz you could wait until the duration of immolate was low, cast soul harvest, cast havoc(with wreak havoc talent) and cast immolate again, potentially gaining a few extra seconds added to soul harvest's duration and then when the duration of those immolates get low you could potentially further extend its duration by a few seconds again, so i doubt it will be dynamically updated and it would add a new gamestyle of metagaming which i dont think is intended, to meit would make most sense if it simply snapshots and thats it.
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you're assuming that destro is supposed to cast spells while moving, guess what? locks arent supposed to, blizz have stated long ago that they want locks to be stationary and tanky which we definately are, anything we get that we can actually cast while moving is a bonus.