Looking further into this as of current, this was replaced with the current system. Previously, SS was factoring into pet skills and was detailed on the cooldowns the same. Whether is was fundamentally wrong initially or simply changed later, it is claimed to be working as intended now, despite still being an active bug being looked into according to the forums. ( but let's just call that laziness)
But you do not gain any significant casting boost by stacking SS on the class and even attempting to do so if you geared purely for SS, would only hurt you overall anyway. If you cannot fit in a full extra cast per 10 (Fester CD) seconds, your benefiting nothing from SS. Go as low as 350 SS or as high as 450, on average you will do less damage overall in builds supplying high amounts of SS, and still will not get an additional cast in per 10 seconds. If you want to take full advantage of what your saying you literally would do Bio2 ->Miasma -> Bio -> Ruin Spam till you have to recast your DoTs again.
If that's your rotation and your not weaving in Rouse, Spur, Enkindle, Ruin2 + Fester, you are already not in an optimal skill rotation. Weaving in these skills, will largely not even touch the GCD when done correctly (have you waiting for the GCD to come up). SS is near useless not because it's not working as intended, but because the way you should be playing the class doesn't benefit from it.