Warning: spoilers ahead for the tenth episode of Legends of Tomorrow Season 3, called "Daddy Darhkest."
Legends of Tomorrow is officially back on the airwaves after a long winter hiatus, and the midseason premiere featured none other than John Constantine himself. Matt Ryan reprised the role that he first played in the live-action Arrow-verse back in Arrow Season 4 when he and the Legends needed to team up in the case of a demon-possessed young girl. While the whole crew did more or less solve the dilemma of the week, "Daddy Darhkest" ended in a way that proved Legends of Tomorrow needs to keep Constantine around.
As a practitioner of magic and experienced exorcist, Constantine first turned up in the Arrow-verse as a pal of Oliver who happened to know what needed to be done to restore Sara's soul to her resurrected body. When he ran into trouble with exorcising a demon from a young girl in "Daddy Darhkest," he tracked down Sara because the demon somehow knew Sara's name. The episode revealed that the possessor was Mallus and that is how he knew Sara's name.
Constantine and the Legends teamed up to try and free the young girl from Mallus, and they were all surprised to discover that young "Emily" was actually Nora, the daughter of Damien Darhk. Sara had to journey into darkness to save Emily (and solve the little issue of getting herself, Constantine, and Snart back from being sent to 1969), and it turns out that she didn't just emerge from that darkness. According to Constantine, a demon's power resides in Sara and it's only a matter of time before she succumbs to it. Constantine recommended keeping the Legends' anti-magic gun handy, and the good guys got a clue that six totems will play a part in defeating Mallus, but something tells me they're going to be in over their heads sooner rather than later. Sara is their leader, and she's not somebody who will be easily stopped if/when she turns on them.
The Legends have their anti-magic gun and the mission to solve the mystery of the totems, but they might always want to keep Constantine's number close by. In fact, if they really might want to keep him around once the situation begins to go south on them. He's the only good guy in the Arrow-verse with the magical knowledge and skillset to potentially save the day.
In addition to his expertise, Constantine has the personality traits to be able to pose a threat to the demon in Sara. He and Sara hooked up in "Daddy Darhkest," jumping each other while stuck in the 1960s. Constantine was genuinely concerned about her when it seemed like she wouldn't wake up after her journey into darkness, and the end of the episode revealed that he's more than willing to have another go with Sara if the opportunity arises. Then, in his very last scene, he recommended keeping a gun handy to take out Sara. He clearly has the spine to do what is necessary, even if it's to somebody he is fond of or cares about.
Besides, the Legends are still down two people after the death of Martin and the departure of Jax. Even Leo Snart left the Waverider after this latest episode. Sure, Wally West will soon join the team, but there will still be room for one more. Why not bring back Constantine and turn him into a regular? Legends of Tomorrow is really the one show in the Arrow-verse in which Constantine would fit as an ongoing character. Arrow is best when light on the superpowers, Team Flash has enough superpowered heroes backing up Barry on The Flash, and Supergirl takes place on another Earth. On Legends of Tomorrow, nothing is really off the table. Bring back Constantine, I say!