I had a exchange recently on a blog with James Creegan (ex Spart, ex BT) where I argued that trying to understand the Sparts as a political group is futile. And what you say here is an illustration of my point. There were a lot of people who were clearly apolitical even while they were in the group - the ones who always shouted about discipline and betrayal but had never read a book. I first started to think about this a few years after I left. I was talking to a friend (an ex-Spart who had been a member around the same time as me) in 1989. I asked him what he thought about the fall of the Berlin Wall, he said something like "I haven't really been following it." This is a guy who spent years in an organisation which never stopped shouting about the defence of the Soviet Union. When the Wall fell and the Soviet Union collapsed he just wasn't interested.
I think the beginning of wisdom as to what's going on here is to be found in Geoff White's 1968 resignation letter from the Sparts -
Geoff White Resignation (you'll find the letter on page 9 after the interview, with a response from Robertson). The Sparts were a purist sect, being correct was the objective, they had little desire to engage with let alone affect the world. This isn't the whole story but it's an important part.
Of course there are those who don't drop out, but continue, like Tom Riley's BT and Jan Norden's IG. I don't know much about the IG but if you look at their journal they are the Sparts of the mid-1970's preserved in aspic - the banners, placards, slogans, articles are identical. Not surprising given who's running the group.
I suppose you knew Norden. What was your impression of him?