Chilling Aura at least demands you make your gladiator agile enough to evade enough hits before the opponent is exhausted, unlike Cheating Bastard which is by far the laziest skill in the game, surpassing even Knockout in that regard.
Shadow Form is actually a very good skill (some deflections still occur but less than before), just not on a par with Chilling Aura at the moment. You have the same problem with Down and Dangerous vs. Armoured Fortress (really Nate, give those Wars a real alternative top-tier skill, nobody uses that D&D thing and for a good reason), Unfair Advantage vs. Cheating Bastard, to some extent even Juggernaut vs. Flurry of Steel - one of the skills is just far better than the other. Furthermore Shadow Form has a rather niche application, it's effect diminishes with the decrease of the protectiveness of the opponent's armour - i.e. vs. other Shadows, Trixters and especially Rages, the skill is not doing all that much. In contrast, Chilling Aura is efficient vs. everything and as it usually happens (and as it should happen) people pick the skill which give them advantage against a wider range of opponents. That's how you build a good Blood Gods-tier gladiator anyway, by making him/her effective against as many opponents as possible.