In contrast with proportional representation there are less obstacles to 3ed parties. If a Progressive 3ed party earned 20% of the vote, they'd get 20% of the seats. People can vote their conscience and be sure to get some REAL representation for their beliefs. With IRV the winner knows s/he won only with the half-hearted support of a 3ed party who held their nose.
While IRV can prevent the so-called spoiler effect, it's unlikely to change the dynamics of our system or its underlying defects.