I don't think it's rare. I have a long list of Fotki's bookmarked that showcase this curl pattern. However, most are new naturals (including me) and have not yet been on their natural journey long enough to acheive BSL+ yet. It takes about 3 years from a person's last relaxer to gain the 12-15 inches required to hit APL-BSL (barring setbacks, breakage, and trims). My guess and hope is that in 2-3 years these women will remain natural and have huge puffs and "long" hair when stretched.
Domenique on Fotki closed her account but had MBL-WL stretched coils. CMW45 and Vibrantthing (both on Fotki) are at APL stretched and growing towards BSL. TruBlu on (BHM) is BSL. Oooop2 (Fotki) is BSL with 4A also. Celeigh (?) on LCHF is BSL too. They're definitley out there.