Proportional representation would remove power from the main parties

With a majoritarian voting scheme we have no choice but to pay attention to a candidate which we don't like and as a result this may mean that we must vote for their main rival. It is not enough to be voting for someone else as this vote may not count, we must balance this risk against the chance that our feared candidate will be successful. With a proportional voting scheme we have no (at least reduced) fear of a wasted vote and this enables votes to be cast on less popular parties, not worrying about having given space to a feared candidate to be selected.