Can't see what all the fuss is about. MacColl never made any secret of his MI5 record – in fact he was quite proud of it and was often accused of romanticizing about it.
My father, because he fought in Spain, was also on MI5 files – as a 'premature anti- fascist' – he was proud of it too, despite the fact that he was blacklisted from work and excommunicated from his religion.
MacColl's 'crime', along with most of the other rank-and-file Communists – (the Communist Party leadership was another matter) and many of the Socialists of his generation, was to be fooled by what was happening in The Soviet Union under Stalin – it took Kruschev's 1956 congress speech to blow the gaff on that one, by which time he was out of party politics. If he was gullible, he was no more so than many of his generation. Nor were they any less gullible than those who more recently supported the McCarthy witch-hunts, the Viet Nam War and the invasion of Iraq (which has included the use of chemical weapons against civilians in these latter two).
MacColl devoted his life to the cause of working people and as one of those working people I've always been grateful for that fact.
As has already been said (by weelittledrummer, he was a humanitarian and also a pacifist, though he respected those who believed that the only way to change the world was through armed struggle – hence his support for revolutionary movements.
He threw his weight behind the Civil Rights Campaign, the Anti-Apartheid movement, CND, the anti-Vietnam war campaign and the Trades Union movement. His last big cause was the miner's strike.
I knew him as selfless and generous to a fault and think myself lucky to have spent time in his company.
MacColl was a great artist and was recognised as such by most people who knew him, even by many of those who disagreed with him.
There is a forthcoming biography in preparation that covers virtually everything that has been 'exposed' in these latest revelations; I for one look forward to the record being set straight.

PS
The Labour Party sold out at the time of the General Strike and no longer pretended to be Socialist - New Labour was an inevitable outcome.
Incidentally, one of his big gripes was the fact that he was never paid for his plays being performed in Eastern Europe.