I feel that Events should be ranked with Kingdom cards because you are making a choice to spend your money on that event over a card for the effect of that event. Sometimes, cards such as Mandarin or Mint are bought solely for their on-buy effect.
I feel that maybe there should be an Event list, but I also want them ranked with the regular kingdom cards. From the games I played, if I spend $6 on Lost Arts over buying a Nobles, I still spent $6, so how does Lost Arts overall compare to a card like Nobles or Goons. I feel placing Events with Kingdom cards at their costs gives a better idea of the overall power level of said events in relations to cards of their costs. Sure we could rank Events on their own, but that does not give a good picture of how strong an Event truly is.