Main queue required for listener block?

[gcox]

Is the main queue used to invoke listener blocks out of necessity or was it simply a convenient choice at the time? https://github.com/mutualmobile/MMWormhole/blob/8fdc7d7e44fc0ca9f1b9d7debf9d013256b5a2f9/Source/MMWormhole.m#L194

I'm wondering if there's any reason why this shouldn't be updated to be user-configurable (besides lack of demand).