Leonie, I don't think it's all that rare, actually. The most conservative estimates from bf advocates (like Jack Newman) put the figure at 1%. That might seem small but it's a higher percentage than that of people who suffer from Type 1 diabetes or Multiple Sclerosis, for example, and those conditions rightly attract both compassion and research funding.
There's a big difference between "diagnosable" and "diagnosed." I believe it's the diagnosis that's very rare, not the condition, and that the incidence of "real" problems is much higher than 1%.
Breastfeeding was important to me, and when my breasts almost completely failed to register that I had had a baby (no changes during or after pregnancy, no engorgement, tiny drops only when expressing, dangerously dehydrated baby, and later, no discomfort or leaking when I stopped bf'ing), I did all kinds of research, figured out that I probably had hypoplasia (where milk ducts don't mature) and presented my consultant with my hypothesis. He examined me and agreed that this was the issue (this was the point where I finally felt he believed me when I said I had no milk) but said I couldn't be officially diagnosed unless he referred me to a breast specialist. I had a 3 week old baby, a post-natal uterine infection, and the ebf ship had already sailed, so I told him I couldn't face going to see the specialist any time soon. As a result, I'm not going to be counted in any "1%" stats of women who physically can't breastfeed.
I'm sure many moms who don't have milk move onto formula without necessarily seeking an explanation for their inability to bf (because they just want to move on from the failure, or because they're embarrassed, or because it wasn't such a huge deal for them). Add to that the fact that many doctors/midwives/lactation consultants are dismissive of women's knowledge of their own bodies and default to the reassurance record ("oh don't you worry, it will come in, just don't give up") when they should be listening (to the fact that there have been no breast changes, there is no leakage, there are not enough wet nappies), and it's not surprising at all that cases of physical inability to breastfeed go undetected. Then, when the milk still doesn't come and the mother finally listens to her instincts, by the next visit the baby is taking formula, and the health pros can just shake their heads and assume that the mom didn't try hard enough / didn't give it enough time. No diagnosis.
I know it's important to keep new moms from panicking about supply and remind them that it takes time to establish, and I know that many women think that they don't have enough milk when they do, or when the supply can be built up with some good advice and commitment. But all the talk about how very very rare real supply problems are (and what was your point in that, anyway? If it's rare should it not be mentioned at all?) can add insult to injury for those of us who experience them.