Why take the real spies back?

It is a bit short on logic, but many reviews of the film state that Alistair MacLean's screenplay favors suspense over logic. And that's what makes it an action classic. It flows well and holds water within it's own structure, but it is really a just an action movie.

"Don't mind him, he's from Barcelona"

In British/American war movies I've seen, the Germans are always portrayed as executing those whom they suspect of spying, with little or no pretence of a trial (e.g. The Great Escape).

It could be the story was written this way to show how British armed forces were morally superior because they still respected prisoners and would ensure a fair trial.

I'm not saying this was necessarily true in wartime, but merely a subtle, maybe even unintentional piece of propaganda on the part of the film-makers.

The question about taking the prisoners back was already explained, in the context of the novel, by the other posters. I'll address the other points.

"Burton even risks the success of the mission by pursuing these men in the cable car - when he straps explosives to it, how in the world does he imagine that he is going to get back up to join his comrades? Does he know that another cable car will be passing him at an opportune moment and, with a wounded hand, he would be able to successfully launch himself onto it and rejoin his colleagues still at the Schoss Adler?"
Yes, he knew another cable car was coming up. Lack of foresight is not something of which you can accuse Smith. He had a lot of little details planned practically half a day in advance.

"And why did the two Germans on the cable car, when they had incapacitated Eastwood, not simply open the door of the cable car entrance and make their escape?"
They were hoping to save their skins by making the "deal" with Smith. They knew he was capable of killing them -- by gunning them down perhaps.

"And why would BOTH of them dare to fight with Burton on the roof of the cable car itself?"
What other alternative did they have? Wait? Two men on one is not unreasonable; they just failed. What *is* surprising is that Smith didn't take his gun. But then again he just expected to blow them up and not have to engage them directly.

"As does the fact that the cable car entrance is deserted when Smith and company are making their great escape."
You're right, that seems surprising. Of course, the castle thought it was under attack at this point -- all hands on deck, etc. Not that it would have been hard to clear the room by the commandos bursting in with the element of surprise.

"Why would the Germans, considering the damage already done to the castle, not simply use explosives rather than hammers to open these doors?"
Two reasons -- first of all, using explosives in stone corridors is an exceptionally bad idea because of collapsing masonry -- second, an explosive in a corridor will always fill it with smoke and render it unusable for a space of time, third, explosives are not necessarily the most efficient at blasting a door. Most of the charge will go into the wide-open corridor unless you tamp it down, in which case you risk the tamp flying off and hitting you. Hammers were the right solution, short of riddling it with automatic fire (which could also be risky due to bullets and shrapnel ricocheting off the metalwork.

That's one of the things that bothers me too - it never really made much sense to me. I think after the big scene where Burton explains all of the plot twists to the Germans the movie goes downhill - everything up to and including that scene is great, everything after a silly if still watchable action movie.

I'll add also that the fact that Burton explains everything to the German officers AFTER his cover is blown never made much sense - why exactly did they need to know that? Unless Burton was just gloating. They might as well have killed them all as soon as the Colonel realized what was going on.

Mr. Rusk. You're not wearing your tie!

Actually, it makes sense. They want to get the German agents in the UK. While they have names and addresses, its always better to have the contacts lure the agents into a trap instead of trying to round them all up at once. And there may be more they can use them for, force them to be triple agents and feed the Nazi intel with false info over time. A turned spy will always try to save their life, so they were not only vulnerable to being turned they could easily be turned and used, especially with the head German agent being dead. Didn't work out, but they had their potential uses that would justify the risk. In the real world, a turned spy is a valuable thing, you can count on him/her to do anything to try to save their own skin and play it straight. Valuable, but also expendable.

And it makes for good drama too.

You answered your own question OP. Drama is the answer. Like someone already said why take the three back for interrogation (makes sense) but let the top-dog Col. insider who could have revealed so much more jump off the plane!

Strictly though it would have been illegal to shoot them without trial, but considering the dire circumstances.

Thought it was because of the wealth of espionage material they can get from the double agents during interrogation. Invaluable to intelligence organizations. But it is a curiosity.

"Does he know that another cable car will be passing him at an opportune moment..."

Yes actually. Cable cars like that are in pairs. One is always going down and the other up. They always pass each other in the middle at the same place. We see this pass a couple times in the film. It would have been simple for Smith to estimate where the midpoint of the cable travel would be and so where to expect the other cable car. Jumping onto it is another thing entirely.

in respect to the cable car i think one goes down, one goes up, so the cable car's would definitely pass at one point.