Look at the war. So far, entropy has been doing very bad. I actually had to play against an entropy deck, and it used antimatter, and I won handily. My deck actually had multiple ways of dealing with antimatter:
A: immolate the antimattered creature. Gets rid of the negative attack, gives you more quanta.
B: if the antimattered creature is a golem, just keep growing it to overcome the antimatter.
C: Just add more attackers to cancel out the negative.
And this is just in one deck. In fact my opponent used antimatter on a golem that had around 15 attack, and I still won by just growing it to overcome the antimatter.
Sure, antimatter has its uses. But it can be countered in many, many ways, which Xinef already listed.