Perhaps the most poignant reason for not engaging in spins in these aircraft is that they were never demonstrated, nor designed, to be capable of engaging in spins. Someone already mentioned the mass outside of center that contributes to staying in the spin, and resists spin recovery. The bottom line is that these airplanes, most light twins, haven't been shown capable of entering anything more than an incipient spin, and recoverying. Why would you underake the project of experimenting to find out the reason?