Hmm. A very interesting question.
First of all, the standard answer to your question, in terms of advice that we (as those trained to teach GMAT prep) give to students studying for the test, is "No. Do not write your essay this way."
However, in the interests of full disclosure: Just to see what would happen, I wrote my GMAT essay similarly to this, where I insisted that the issue was more complicated and elaborated on that basis. And I received a 6.
What to make of these conflicting facts?
First of all, your essay is extraordinarily well-written, as I think my own was (I was a literature student in college and am a Ph.D. student now, so I am a very well-practiced expository writer). That does matter. You give tremendous examples and tremendous elaboration. If you really did do all this in 30 minutes (starting the timer from when you first read the prompt), then kudos to you, and I think you'll do fine on test day. If you didn't ... be very wary. Starting off this way and having to cut yourself short due to time will result in a low score, as opposed to the tried-and-true formulaic pro or con essay that, even if it's short, will always score a 4 minimum, a 5 usually, and that's good enough for anybody.
Second of all, despite committing the cardinal sin of "changing the topic," you really do make an effort to stay on topic. You state correctly that you
disagree with the prompt because the prompt says "x is always true" and you believe that "x is sometimes true." IF you'd written this essay simply to say that "well, sometimes yes, and sometimes no," I'd say that you never took a clear stance on the Issue and I'd give you a 4. However, by consistently tying it back to their prompt and reiterating that your "sometimes" answer was meant as a response in disagreement with the prompt, you avoid this pitfall.
So, my conclusion is this:
Yes, you can write your essay this way, in theory. This particular essay is a clear 6.
DO NOT TRY THIS AT HOME if you cannot do just as good a job as ohfred did here, and do so in 30 minutes, in terms of clear writing, clear examples, and solid logical reasoning in the essay body.
The essay is ONLY acceptable because, despite taking a middle-of-the-road position, it continues to reference the topic and a clear stance on the topic. If the same essay left out those key sentences, it would drop immediately to a 4.
Finally: If it ain't broke, don't fix it. On test day, even to you, ohfred, I highly advise that you pick one of the two clear, extreme sides. It's just easier to do so. No, you don't "have" to ... but unless you're strongly compelled to attempt what you did here, why risk it? If you can do this so well, you can certainly do that well. The Green Bay Packers of the 60s ran power-sweep left and then power-sweep right. No one ever called them creative. We just called them Champions.