You're really supposed to use these buffs out of combat before engaging a mob. As a rogue you have time to clear multiple mobs between casts so it's not even an issue. I think a better solution would just to make all buffs last 15 minutes. That way you don't have to waste time and energy casting and recasting. They would still have to be removed on death obviously.
The thing is this would be a nice solution for all classes. I know my druid struggles to keep bark up due to the amount of time it takes to clear mobs, so making it instant cast would allow me to save myself while in combat. If these skills for rogues were made instant, so should they for every class.
This is all good if otm don't change the current cooldown, if they make it longer cause they increased the time it last .... Would never be able to cast it on anyone
On topic, a few sec count in lixs, for duel devicers the cast time puts en off, and aoes interupt those skills making it harder to cast