They would be very, very useful. We have some ugly replications happening to translate all our annotation in to polygons which Collector can display.
If you publish a layer that has labels enabled from Desktop in to an AGOL hosted feature layer, add this to a web map, take the web map offline in Collector, then open the web map you'll see labels. Obviously not something to rely on, but a handy bug that shows Collector 'supports' labels.