The important thing in a crossover is to decide which universe is predominant, and keep it consistent throughout the story. Yet the world of Hercules and Xena is full of fantasy demonic things. Yep, this is just one of many things on which Keith and I think as one. But the Highlander world was originally established as Just Like Ours, except with Immortals in it, and the sudden appearance of demons, magic stones and psychic witches is jarring. That's part of it, definitely.
Younger characters exist in a state of flux.
They haven't chosen a life path yet, they're still developing emotionally and morally. They make mistakes that a more experienced character would never make, allowing for more story possibilities. And if done right, a glimpse of a character's younger self can provide a valuable insight into his present self. The writers on Highlander do this all the time with the flashback sequences. There can also be a strong element of irony in such stories, when you know what the future holds for a character.