After the update it won't be useless anymore. Evasion is nice but if your going for full offense it'd be the area to hit to increase the effectiveness. Couterstance doesn't improve defense unless your directly targeted. It does improve offense because the counters also cause damage.
Counterstances effectiveness is moot both offensively and defensively unless your face to face at close range being directly targeted. With the proposed changes the defense might be extreme. Consider the fact that the defense down effect (penalty) exist regardless of the attack being direct or not. So if the enemy is attacking you directly face to face then you can benefit from counterstance under the current setup. On the other hand if it's attacking a tank or another target you only have the defense down penlites without the benefits of counter.
With the changes listed the defense penalty won't be the same. We don't know what the change will be but considering it only gives vit bonuses now I can assume it's at least 3/4 defense reduction and severe enough to make it extremely dangerous to use. Maybe not even worth the risk. This would break it enough to the point of worthlessness in most situations. Simply put the balance of lowering a useless trait changes and this single change could break it's effectiveness.