There are useful enemies that make you fertile and sterile enemies that wear you out.
I like that formulation. I like the way it limns the subtle concept of an intellectual sparring partner who brings out the best in you through integral opposition while keeping you from slipping into the easy tendency toward moralistic tunnel vision, where winning the argument and trouncing the enemy at any cost becomes the goal. Maybe that's not so remarkable an attitude among scientists, say, but I find it to be rare in general culture. There are a handful of people I describe as favorites "to think with" — they manage to enlighten even as they provoke; they pull off the difficult trick of balancing stunningly original insight with clear, almost obvious, expression.
Useful enemies don't have to be individuals, of course; they might be general concepts or themes. One theme I've been circling around for the past year or so is the superficial progressive obsession with achieving racial/sexual demographic parity in artsy fields. Well-meaning folks, I'm sure. Nothing more than guilt-ridden honkies looking for a simple, clearly visible metric to reassure themselves that they aren't accidentally contributing to someone's oppression. Their vapid "activism" is the sociopolitical equivalent of feng shui — rearranging society's demographic furniture in order to, like, free up the inequitable energy flow and create good vibes of social justice. But though I mock them, I'm glad for the time I spent taking their arguments seriously and engaging with what they said instead of dismissing them after a cursory glance. In the spirit of Nietzsche's rules, they were the stones against which I sharpened my own thoughts: why would I resent them for that?
That theme could easily become a sterile enemy, though, if I let it. It could just be something I routinely return to in order to reenact the same old drama, with nothing new to say, both of us going through the predictable motions in a dysfunctional relationship. Especially on the social web, I feel, there's a phenomenon where one has to perform one's dislike, even when there's no point to it. It's not enough to quietly ignore people or websites that waste my time; I have to announce my ignoring in a passive-aggressive stage whisper to someone else. I see people in forums and on Twitter doing that all the time: seeking attention in a symbiotic relationship with someone they profess to be bored or sickened by. It's like watching kids who think they're cleverly disguising their flirting by punching each other and calling names, only it's pathetic instead of cute. I've noted this unhealthy dynamic a couple times recently, and it motivates me to pay closer attention, to be sensitive to when it's time to add a period and turn the page.