Sorry, I did not have a specific example when I made the question, but now I have found in Book 2 of the Odyssey two examples that are related with this.
58. μαψιδίως: τὰ δὲ πολλὰ κατάνεται. οὐ γὰρ ἔπ᾽ ἀνήρ,
59. οἷος Ὀδυσσεὺς ἔσκεν, ἀρὴν ἀπὸ οἴκου ἀμῦναι.
345. δικλίδες: ἐν δὲ γυνὴ ταμίη νύκτας τε καὶ ἦμαρ
346. ἔσχ᾽, ἣ πάντ᾽ ἐφύλασσε νόου πολυϊδρείῃσιν,
347. Εὐρύκλει᾽, Ὦπος θυγάτηρ Πεισηνορίδαο.
In both cases, the sense requires from εἰμι a durative aspect. But since there is no imperfect vs aorist distinction for εἰμι, the iterative form of the verb is used instead to denote the durative aspect. So I wonder if by default ἦν is more like an aorist than an imperfect, and here is used the iterative form to denote the durative aspect, because otherwise it should be not durative, like an aorist.
In addition to this, there may be other aspects beside the durative that are usually denoted by the imperfect or the aorist. Could be that in those cases another verb is used instead of εἰμι, like πέλω or γίγνομαι?
Well, may be I am divagating too much with this