Honestly I think the card needs a change of some sort. Will lowering the cost change that people favor direct damage vs damage over time as someone said already? No.
Yes it has it's uses, but against the AI very few if any decks use it, and PvP it is not that great either for this card.
I have played PvP decks with Plague but honestly the card is way underpowered. Maybe if the Plague spread to other creatures when a creature died then the cost would be worth it. It would still be damage over time, but it would be a big change, infact I would use it, against falsegods even.
Also a plague spreading rather than needing to use another card makes sense.