Thats ignoring that Isperia is as much about preventing attacks as gaining from them when it comes to weenies and tokens. Isperia ensures you opponent will have to consider if that 1 or 2 damage is worth the card(s)
That's exactly the point. If Isperia would prevent the attack, then Detain was unnecessary. And if Isperia would have benefited from the attack, then Detain was counterproductive. Either way, they generally work against each other.