you make a good point, over-use of anything can be toxic, even deadly (alcohol, tylenol, sweet n' low), but you make it sound like Senna is an essential nutrient. And that's completely incorrect. Senna is NOT an essential nutrient. There is no reason for you to put Senna in your body, period.
If you use it for occasional constipation relief, then there's no problem with it. But the keyword in that sentence is OCCASIONAL. There are ton's of scientific studies and clinical studies that point to excessive use of Senna can make your colon become dependant on it and you will not be able to have a Bowel movement, unless you take more Senna. That fits more into the definition of an additive drug, than an essential vitamin.
In the case of Senna -- Excessive Use = more than 7 days!