Everything is parallel. Prospero’s dukedom is usurped, and Caliban’s island is usurped. Caliban, with the incompetence of Stephano and Trinculo plot Prospero’s murder.  Antonio and Sebastien plot Alonso’s murder. Caliban is forced to do trivial jobs (getting sticks, moving longs), and Ferdinand moves logs.

Why are all these characters in parallel situations? Perhaps it is to make us as the audience question character motives. In Caliban and Prospero’s case, it makes us see Prospero in a less “righteous” way. He’s not completely in the right, because he commits the same crimes as the ones that happen to him.

In the case of Sebastien/Antonio and Stephano/Trinculo, it puts the two nobles in line with the bumbling drunks. It puts their plans of regicide on the same level as Stephano’s drunken fancies. The comparison serves to make the idea of regicide look like a folly idea, akin to one that would be made unde the influence.