He's a likable character, and often quoted.
That Monty Python could make fun of him in the 1970s in such a fashion was a bit prescient. This sort of political tripe was known, but not so common then. I suppose it was more common in Britain, which had had a strong socialist strain since George Bernard Shaw and the Webbs, Sidney and Beatrice, before WWI. It ties in a bit with my recent post on the misunderstanding of war in previous eras. Dennis's views, while still widely mocked as an exaggeration, are more generally accepted now. This is because MP is also making fun of Arthur's reasoning in the scene, as they do throughout the movie. They aren't entirely wrong, which is what makes them funny. Yet in the end they make fun of all references to honor, to God* to patriotism, to good intentions. It's their job as comedians and satirists. Yet when one totals it all up, they make more fun of one side than the other. Which, when you think of it, is better than SNL and late-night comedy has done over the same period.
It is not merely that such ideas were unknown then, but that they were 100% unworkable in that context. They aren't especially workable now, but you can extract out bits and make them work in highly cooperative societies with enormous amounts of social capital, like the Scandinavian societies, where everyone looks like second cousins. And insofar as the newcomers don't look - and especially don't act - like the traditional society, with ties and affection built up over hundreds of years - they aren't doing so well there.
We now live in a 55+ co-op. We own the land collectively, but each owns his own building. We distribute the water bill. It's a private community so we hire a team to collect the trash, snowplow, cut down trees, maintain the water mains, etc. We screen heavily for income and attitude to get in here, and 55+ communities by their very nature don't have a lot of violent crime, use of schools, drag racing and the like. It is about as solidly weighted toward making shared property work as one could imagine in America. It largely does work. Yet even we have significant disagreements and have to sometimes tell a minority to just stuff it in favor of a majority decision. The myth that some more thoroughgoing socialism could work across an entire nation ignores the realities that it just barely works under the most favorable conditions. Hothouse flowers are pretty, but don't transplant well.
* I first saw the movie in 1975 with two highschool friends. The female friend was deeply offended by the bias against any reference to God, noting accurately that at one point when Arthur says "In the Name of God" he has shit dumped on him. I thought she was ridiculous. In retrospect, she was right and I was wrong.