I teach at a university.
Last week I had my students doing a class debate with a twist- they were in two teams, and each team was a different theoretician; they had to argue from that person's point of view. The two theorists are opposed, so it turned into a "defend your turf" war.

One group had a guy in it who rattled off his group's whole presentation in under a minute, and then couldn't resist showing off his knowledge by going into a rant about how much he, personally, disagreed with it, and how the theory was bollocks, etc. This to the horror of his group, who now had a traitor in their midst, and the delight of the other group, who were cheering their spy on. Loudly.

One of my co-workers was talking to me afterwards and asking how the hell I managed to make a class full of 20-somethings laugh so loudly they could be heard two classrooms down. I explained that, for the good of the truce, we had imposed a speaking ban on the traitor so he wasn't allowed to chip in until I let him speak. So by the end of the class several people were almost in tears at the fact that this poor lad was visibly shaking with the desire to join the fray....
(Sivvus trolley collision in the fruit aisle led to a jam, Sun 9 Dec 2012, 14:32, 2 replies)

I wish I could have been there.
My uni career was filled with frustration with overconfident students sending perfectly good lectures off on tangents. The worst was that when I attempted my own non-sequitur inquiries the result was rolling eyes and a pained silence only broken by "Are there any more questions before we get on with this topic?"
(the other guy not the one that gave you coochie warts, Sun 9 Dec 2012, 16:31, closed)

To be fair, it's one of the things that goes with the turf
We can't fault people for wanting to get involved, but as you say when it derails the topic you have to be careful. I would only try a trick like this with someone I know wouldn't be discouraged by it; if one of the people who are generally quieter started a tangent I'd be more than happy to let them get on with it!
(Although personal opinion = // = theoretical debate *shudder* )
(Sivvus trolley collision in the fruit aisle led to a jam, Sun 9 Dec 2012, 16:43, closed)