Hi pookee,
You wrote: "Intensification of mental defilements" seems dangerous enough, but why would this occur?"
I cannot speak for Ven. Khantipalo. Dangers I see (solitary practice or not) are: "losing touch" during A&P, taking manifestations of "energy" and pretty deep insight somehow at face value or for granted, not seeing through them (which does require mindfulness, among other things). There's a list of ten "defilements of insight" which I'd associate with the "dangers" of the A&P. Extreme "cases" of A&P "danger" would look like the symptoms of a manic episode: losing grasp of finances, career, relationships. There's a page on this aspect of the "dangers of meditation" here:
http://dharmaoverground.wetpaint.com/page/Bipolar+Disorder+and+Insight
Then there are the "dangers" of the Dark Night, the dukkha nanas, depicted in many passages in MCTB. And again, mindfulness is necessary to see through these things.
All of these can be viewed as dangers to be aware of, but just as well as challenges to be met with appropriate means. I think both are necessary: knowledge of the potential pitfalls, and knowledge of how to deal with them. In the absence of a teacher, the maps are indispensable for the former, in my opinion; much more useful anyway than dropping dark unspecific hints about dangers.
For one, dark unspecific threats will have the effect of only encouraging people of a certain mindset to try regardless.

You also wrote, "how does this differ from the normal arising of strange reactions during meditation?" - Intensity? Failing to even keep them in mind (mindfulness!), total failure to investigate them at the sensate level, totally buying into their "content" instead?
Nice topic!
Cheers,
Florian