Many people have never heard of HPPD, or if they must seek treatment they are too embarrassed to admit to having used hallucinogens, and therefore the statistics on its prevalence are probably incorrect. This is another good reason that hallucinogen use should be legal or at least socially acceptable: research could be done, and people with disorders of this sort could get the treatment they need. Luckily, many people don't need to get treatment, because HPPD often fades over time of its own accord.
Now, don't be like me and assume that your brain is broken just because the rainbows around lights at night seem suddenly really noticeable. HPPD is only diagnosed when the disturbances are so severe that the patient cannot ignore it. Like other psychiatric syndromes, a requirement for diagnosis is that it impairs normal functioning. To be fair, I had no idea that light halos were a common occurrence until I asked my boyfriend, who verified that he saw them too. :)
HPPD raises some interesting questions about how drugs affect us. One common theory is that hallucinogens permanently affect the nervous system, creating less sensory gating (like Huxley described in The Doors of Perception). This leads us to be more open to receiving sensory signals that have always been there, but are usually filtered out. There was also an interesting study that I read about that showed that those who use hallucinogens have a lowered ability to distinguish between colors, and to distinguish between a very fast strobe and a continually-shining light. I found this to be the opposite of what I've experienced: my drug use has made me appreciate color so much more, and when I'm high I can even see the strobing in cheap fluorescent lights, which is super annoying. Anyone else have a different story?
I was also kind of surprised that HPPD only includes the visual aspect of hallucinogens. It seems to me that other aspects of a trip could last too, but I guess people with that are just called "crazy." :)