Dieing based on USE flag combinations isn't allowed. A default OCR engine should be chosen and if none of the flags are specified it should fall back to that. http://devmanual.gentoo.org/general-concepts/use-flags/index.html
Thanks for the report! And sorry for the delay I've just added 3.6.0 in tree, which defaults to ocrad (as suggested upstream)
*** Bug 254751 has been marked as a duplicate of this bug. ***