If I'm gonna have both a causative and a passive, I'm going to also have a passive of a causative. I'm not gonna have "teach" and "is learned" without "is taught".
My deverbal adnominal can also be substantivized. So in a way it's like a participle. So, if I only have an active participle, then I have no means to derive, say "word" or "addressee" from "speak" (spoken > something spoken > addressee/word). On the other hand, without an active participle, I have no good way to get "writer" or "writing impliment" from "write" (writing > something writing > writer/pencil).Why?
I arrive at the same conundrum with action nominals, after all "destruction" can mean either "destroying" or "being destroyed".