Debates often need to expand into related areas, so as long as theres some sort of relevance such that it can lead back to the original topic.
Where a debate topic gets braided into multiple miniture debates it can lose its purpose, although someone may have a point relating to a previous item in the debate that hasnt been mentioned in a while. In this situation someone may try to change the locus of the debate, and so if what they have to mention on the previous sub-topic is important enough, they can succesfully acomplish this as long as the can finalize the current sub topic, though it might take a few posts.
As limey said, it depends on the nature of the topic.