Exactly. Fentanyl is one of the most potent opiates which is even somewhat safe for human use, 50 to 100 times more potent than morphine, depending on cross-tolerance and other various factors. Thus, what better way to use it than in a safe I-Doser dose?
Oxymorphone would be for those who want a different opiate buzz, although oxymorphone is stronger than heroin as well.
I just feel that, as my favorite class of drugs, opiates could use a bit more representation.