You're talking about Poison and Psychic as if they're exchangeable on a Pokemon or something. You're also making the comparison using just a single type (Light) as justification. Steel is a better defensive typing than Rock, but that doesn't stop people from using Tyranitar or Aerodactyl. At this point, any boost to Poison at all will save it from the pile of uselessness it was before. Just because Psychic does the job better doesn't mean Poison isn't viable.

When I said that Dark isn't offensively oriented, I was talking about its moves. In canon games, the highest base power of a Dark-type move is 80. Instead, a large portion of dark moves are based around statuses and unique effects. Type coverage for dark-type moves isn't exactly sublime, either. Also, we have a move called Black Hole, which is basically a dark-type Explosion. It would be broken if Light wasn't immune to it.

I stated in a previous post that, if the 12-gym idea is implemented, Fernando would be the first to go. He has the least established character out of the gym leaders. Mandy, at least, is involved in the plot around Hollowlog, and that propels her far beyond Fernando in terms of importance.