OK, since I was challenged on this :-).
This is a study on binding of various anti-androgens to receptors.
"The Use of Human Skin Fibroblasts to Obtain Potency Estimates of Drug Binding to Androgen Receptors", Eil and Edelson, J Clin Endocrinol Metab 59:51, 1984. Its old, but Spiro's been around forever it was studied back then.
He we learn that spiro has a great affinity to bind to DHT receptors, but it does not bind as well as DHT itself. The affinity of DHT=100% and the affinity of spiro is 66%.
Spiro of course as the effect of eventually cutting T at the source (the testes).
So, if your T is low enought that conversion of T to DHT doesn't produce a high level in hair tissues, spiro will have blocked many of the DHT receptors before DHT even gets to them.
But, some amount of DHT WILL GET THROUGH to the receptors, and depending on the sensibility of your hair to DHT and how miniaturized it is, it will continue affecting the hair.
So, if you had not much thinning prior to HRT, stopping all DHT will not be that usefull. But, for someone who's had major thinning (say that they will go bald by 30 if they don't arrest it), I think its usefull. For others, generic finesteride in an effective dosage (a small fraction of Fincar pill) is so cheap ($50 a year!!) that the companies are sure not making money off it. The worse side effect of using DHT blockers are breast enlargement and loss of male libido... LOL, I don't care.

Anyway, I'm taking it and I've looked all my hormones so much that I should apply to a medical school or teach it.