It enables an instant win combo with Vraska, Betrayal's sting since Vraska enters play with 10 counters and the -9 ability gives 18 poison counters.

Instant-win combos are bad for MTG since they make the game not fun for the other player. Combo drawn and now the game is over is not fun at all.

So do your job Wizards of the Coast and start banning cards until all the unfun aspects are gone. Contrary to the dumb idea that bans don't fix anything, bans do fix everything and you should be banning much more cards.