It was rather an unstated assumption that the arguments have not enough merit. Of course a troll can make few good arguments, and perhaps there were few in those threads, but most of the debate wasn't centered around reasonable arguments.

The worth rebutting property itself can possibly be further reduced with respect to the motivation of the debater. Arguments may be rebutted for pure intellectual curiosity, just as "let's look whether I can find a hole in that reasoning", they may be rebutted to ensure that other people don't fall victim to fallacious deductions, or they may be rebutted to show one's intellectual superiority or other social signalling purposes. Those are different mechanisms and an argument worth rebutting for one reason may not be worth the same for other reasons (in fact, presence of the signalling reasons depends more on the context than on the argument itself).

I don't endorse the rebutting for signalling, as it is in fact what the trolls generally aim for. When other motivations are present, a better way is to create a separate post to discuss the problem, ideally after waiting for a while until the troll disappears. Instant rebutting will likely be contaminated by trollish distractions and thus suboptimally productive.

That hypothesis is actually the most reasonable one. The troll's arguments didn't have any merit - they were all perfect examples of every bad argument going - but they were the arguments one sees time and again from people who aren't (consciously) trolling.