yeah, it only works when casting instant cast spells. there are bosses out there that die so fast that you don't need to conserve mana and just go ahead and spam ruin II such as demon wall, afaik, there is no penalty for being in melee range for demon wall, it's good knowing that whenever you cast ruin II, it is doing 30% more damage via book attack, or when casting bio, you get a free initial 65+ damage on cast.

free 65 damage when you are gonna use ruin II to fester on longer fights, free 65 damage whenever you cast bio, or free 65+ damage whenever swiftcasting a spell.