pster wrote:But why would it use secondary endings?
Again from Smyth (805), "Many verbs have no active future, but use instead the future middle in an active sense."
I don't pretend to know why with any certainty but I would think that it has to do with the fact that the subject is participating in the action of the verb. In Modern Greek, the verb "to be" has secondary endings in all tenses. Instead of εἰμί, it is είμαι.