'A single word sums up the master function of philosophical practice: "to draw a dividing line between the true ideas and the false ideas" ' Althusser, Philosophie, 'Comme arme de revolution' in Sunil Khalil, Arguing Revolution, p114
You asked 'how is he any worse than Marcuse'. Well, there is lots wrong with Marcuse (he was, as the Maoists say, an agent of the CIA-forerunner the OSS, in the forties) and I couldn't better Paul Mattick's Critique of Marcuse. Still, Marcuse's response to '68 was qualitatively different from Althusser's: Marcuse got stuck in, probably coming across like the oldest swinger in town, but he was there in the teach-ins and the protest.
Also, you take issue with my citing 'another philosopher', Castoriadis. Well, you could fault him, too, I guess, for being a bit of a gadfly. But it is hard to avoid the obvious, that Castoriadis was on the right side of the barricades, Althusser on the wrong side. I mean, literally. Castoriadis' and his comrades were deep in the student protests, and the workers protests that followed. His Socialism ou Barbarie group is today acknowledged as the intellectual inspiration of the student revolt.
By contrast, Althusser supported the PCF leadership, that sent its goons to break up the protests, tried to pitch stop the workers from joining the students, with an appeal that was pretty close to the 'hard-hats' attack on the students' protesting outside the Democratic Convention.
When I was at college, there was quite a revival in Marx studies, and some excellent exegetical books written to restate the core of Marx's argument. The authors were Robert Paul Wolff, Chris Arthur, Paul Mattick (Sr and Jr), Roman Rosdolskly, Geoff Pilling, Anwar Shaikh, Jairus Banaji (who translated Henryk Grossmann's book on the law of accumulation), David Yaffe, and more latterly Tony Smith, Sean Sayers - none of whom had any time for Althusser.
By contrast, the contribution of Althusser's 'Lire le Capital' to anyone's understanding of Marx's Capital was nil. When he wrote, in 'The Future Lasts A Long Time', that he had never read Capital all the way through, the Althusserians threw up their hands in horror, and said that he must have been mentally unbalanced to have said such a thing. By contrast, all the people who did read Marx's Capital thought, 'yes, I always thought that must have been the case'.
Rather like Derrida's laughable meanderings on Capital, Althusser came across like a philosopher, who was not really interested in Marx's Capital, but only in using it as a backdrop to his own 'methodological' word play. The understanding of Capital - either the book or the thing itself - did not advance one inch. The formalistic multiplication of levels of analysis was just obscurantism, writ large. The preoccupation with base and superstructure, one big distraction from analysing social reproduction.
His one great claim, the epistemological break between the young, humanist and Hegelian Marx, and the later 'scientific' Marx, was simply wrong. No-one who seriously studies the mature work takes the 'epistemological break' seriously. The best Marx scholars (and they are not perfect, but they are most definitely serious) like Tony Smith or Chris Arthur have no problem acknowledging Marx's debt to Hegel, because Marx had no such difficulty, either. Althusser's war on the Hegelian residue in Marx turned out to be a programme of taking the meaning out of Marx.