What I think is Riot's reasoning behind Morellonomicon being core on most mages

is bought on mages 90% of the time, since it's an excellent source of AP, CDR, and Mana. However, most don't get it for its passive, which applies Greivous Wounds. I believe Riots reasoning behind putting such a unique/niche passive on such a generic/core item is this: Riot wants mages who have to be able to take out about 60-70% of their health, then the GW passive will keep the enemy from healing, and then they could go for the kill. Remember it reduces all forms of healing, so a or ult could be made nearly useless, and I'm pretty sure it reduces base health Regen as well.