How important is a cortina, apart from clearing the dance-floor between tandas?
Does it affect the mood and energy of the milonga?
Might it complement or clash with the preceding music?
What is the ideal length of a cortina?
Is it a problem if the same cortina is used all the time?
Should the DJ bother with this, and just focus on the tandas?
Well, I'm going to have a little gripe. I wish more DJs would indeed consider the contribution of the cortina. I've seen the mood created by the most thoughtful selection of music spoiled by a cortina which is soporific & energy-sapping or jarringly inappropriate.
Cortinas which allow dancers to not only clear the floor, but are also long enough to return to one's seat, have a drink and reconnect with those around, are considerate of the dancers. So, nowadays my cortinas are usually around one minute in length, and that seems to work well.
Personally, I like using a variety of cortinas to complement or spice up the mood during a milonga - not too gloomy or bland. On the other hand, the Heavy Metal cortinas which I heard some years ago in Buenos Aires, of all places, were way beyond the pale. Some current favourites: Pretty woman (Roy Orbison), Words of love (Mamas & Papas), Son of a Preacher Man (Dusty Springfield), Back to black (Amy Winehouse), Hard times (Ray Charles), Breaking up is hard to do (Neil Sedaka).
Am I being too precious? Does the cortina really matter at all? Next time you're at a milonga, take note and decide for yourself.