Posted to a couple of forums I read:

Having seen the movie four times, I now think the Canto Bight scenes are integral to Finn's story arc. I call the sequence "The Last Temptation of Finn." The Force Awakens establishes him as a sort of conscientious objector fleeing from the First Order, but he's not yet committed to the Resistance when he, Han and Chewie infiltrate Starkiller Base. He's there primarily to rescue Rey, who was only taken prisoner after he effectively abandoned her at Maz's castle. Finn could be motivated by romantic feelings for her, or maybe he has a guilty conscious over leaving her side at Maz's castle.

At the beginning of The Last Jedi, he is still uncommitted to the Resistance but remains motivated by his need to protect Rey from danger. The Canto Bight mission presents him with temptation, a beautiful, opulent setting that's far removed from the grim situation the Resistance currently finds itself in. If Rose wasn't with him, it's possible he would've slipped away unnoticed into the crowd, and that could have been the end of his story. After all, why go back to the on-the-run life of the Resistance when places like Canto Bight exist? (This is not unlike the Las Vegas casino scene in the first Percy Jackson movie, which I just recently watched with my daughter. Las Vegas turns out to be the Lair of the Lotus Eaters, and it nearly derails the hero's quest with its intoxicating temptations. Canto Bight is the Lair of the Lorus Eaters In Space. Is it a coincidence that the master codebreaker they're after wears a red rose brooch on his lapel that kind of represents a-- wait for it-- lotus flower?)

Instead of following that path, Rose appeals to Finn's sense of morality and justice, and maybe he sees his own Stormtrooper past reflected in the slave children tending to the fathiers. We meet DJ, the amoral, politically-unaffiliated codebreaker completely unconcerned with the First Order and the Resistance. DJ represents to Finn the type of person he might one day become if he follows this path. DJ is one path, Rose is the other, like the devil and angel on your shoulder, battling for your attention. As they travel back to the Resistance fleet, DJ gives Finn some advice: "Live free. Don't join." Finn's expression indicates he hasn't completely dismissed the notion yet, even at this late point in the story.

Later, Finn is forced off the fence when DJ betrays them to the First Order and endangers the fleeing transports. He's possibly motivated by the same guilty conscience that developed after endangering Rey on Takodana, only now it extends to the entire Resistance. As he defeats Captain Phasma, he finally identifies himself as "Rebel scum." On Crait, he gives a pep talk to the assembled troops before they head out to the battlefield. And he even attempts a hero's suicide by flying his Ski Speeder headlong towards the still-charging Battering Ram Canon.... only to be twarted by Rose, the angel on his shoulder, who may have developed her own guilty conscience after she helped put him on this path.

In other words, I like Canto Bight! It presents a fork in the road to one of the main characters, and helps him grow into a hero and leader that the Resistance currently finds in short supply. And it's totally in keeping with Star Wars' mythological roots.