Maybe that was what attracted me about Maclean. As I see it... he did it more because he wanted to follow his friend, Philby in this case, and later on felt as lost and Burgess was... don't really now how to express it.
It can be that I like the Maclean I think he was and not the one that actually is shown in the miniseries... I just recall how he suffers when his father is dying and Philby is pushing him to make a choice (he obviously does have the beliefs Philby has but he doesn't have his passion). I like to think about Maclean as the guy behind the guy... he was going to be there for Philby because for him their friendship was important and it was based not just in that relationship but also in their common beliefs. In the moment the become four he is also there for the other two, especially Burgess and more toward the end. That's is why he is hurting so much when he starts feeling jealous of Philby and his wife (with good reasons later, anyway)... I'm not sure it makes sense all I'm saying.
Yes, he is the weakest in beliefs but I think he follows them because he feels their passion and because he value their friendships... just an opinion (maybe he just let's himself go for a better thing to do, but I just can't think those were his motives, that's all).

I think you're right that he seemed to be more of a follower - so stronger personalities like Philby and Burgess could sway him one way or the other. And then when he was too far gone to leave "the game", he realized what happened (I"m only speaking about the movie here, mind you - I think IRL they were all quite different characters from how they were portrayed ...)

Anyway, of course, I very much love the entire series, but it's mostly Toby for me in "Cambridge Spies." ;)

For me are both of them. I love Toby because he makes a great Philby, all charismatic and so sure of what he's doing... I discovered Rupert in CS so it's kind of special for me ;)

I'm sure these four men were quite different in real life, of course!