After some time, I came to appreciate Johnny a lot. With him, you can:
1) adapt - obviously
2) use 5 golds per game with Ciri, or get rid of likely "dead" gold like Ciri or Zoltan when playing against Radovid or if they came in the 3rd round
3) anticipate opponent's moves: As soon as I get a glimpse on the opponent's strategy and can decide which card is less likely to play (e.g. ST and 1st 2 moves are no dwarves - Igni is less useful), I sometimes already use Johnny. Got opp's Isengrim? He is nearly a waste for my monsters but at least I know what to expect in the next round (e.g. I can't tactically pass for fear of being overpowered with Isengrim in the next round).
One case where I decided not to have him is when playing ST with Broover. Because Broover has access to all silvers, it makes less sense to use Johnny on silvers; the silver slots must be powerful game deciders and with immediate action.