I had an anti-Obamacare Republican canvasser show up at my door during election season, and ended up debating for about an hour on the front porch. Though it was an enjoyable and intelligent discussion (far from the stereotype of the Tea Party), I couldn't help but realize that I effected a 1-to-1 "time attack" that drained his ability to canvass voters who actually had a snowball's chance of voting for his candidate.

For better or worse, that seems to be an attack vector in a free marketplace of ideas: finding ways to burn the time and energy of your opponent. (See also: "outrage fatigue".)

This is one of the major goals of trolling -- trying to get someone to waste a lot of time trying to convince someone who isn't actually interested goes side by side with trying to get someone to waste a lot of emotion on someone who doesn't actually care. Part of what makes it so effective is that it can be hard to distinguish from genuine concern (ie, you found it an enjoyable and intelligent discussion, but a troll might act the same as you did purely to waste the canvasser's time and not actually care about the topic at hand.)

One of my rules is that, if someone seems to be trying to get me to invest considerably more time or energy than they're investing, I'll only engage if I find the process of researching/writing about a given topic valuable in and of itself.

These days I usually write about things I want to understand (through writing), not things I already understood. I like to thing this approach is immune to trolling.