I think it’s unethical to tell Fringe groups like socialists and Libertarians that approval voting will help them. Looking at the distribution curve of political ideology a majority of people are moderates somewhere between Republicans and Democrats. Socialists and Libertarians will do worse under approval voting since at least in FPtP there are so few candidates in general elections after a primary they can hope that a major party nominee imploding will let them squeeze in.
The best system for Fringe ideologies is obviously Proportional representation with lots of seats in winning pools. Approval voting will harm not help 3rd party crazies we should be honest with them about that reality.