Bottoming out for nitrates and BGA tend to go hand in hand quite often. When letting nitrates drop below 10ppm which is exactly what you said you should do, you run a higher risk of developing BGA. I learned that from Aqua. You would vigorously have to keep your eye on that as well with your method as you stated nitrates of 6ppm or lower.
I always tell people that are new to planted tanks to keep nitrates at 10-20ppm and phosphates at .5-1.0ppm. This is a good standard for most hobbiest.
But when running a tank with high light, CO2 or liquid carbon, and a good dosing plan and in my case a 220g tank with over 2/3 of the tank planted with non-green plants that actually use alot of phosphate. And in order to attain more non-green color nitrates need to be low, 10ppm.
I've never expierenced diseases caused by dosing and that is what I was asking... what diseases/problems (not algae) have you expierenced by dosing higher phosphates? I have exceeding healthy plants and I'm sure people dosing El which puts higher amounts of ferts in their tanks also have healthy plants that are in no way prone to disease. This is where we disagree.
I get color at a low phosphate reading but is nothing compared to when I increased dosing over the past few months and I found that non-green color becomes more intense with higher levels of phosphates (in my tank which has high lighting and low nitrate). This causes no harm to plants, which is what I getting at and your saying jeopardizes plant health. Any hobbiest that custom doses their tank or uses El are going to monitor their tank levels closely. Same with keeping a low nitrate level which you abdocate and would have to monitor closely.
You advise against my method but other than using Aqua's example which is referring to BGA and not the plants and saying it jeopardizes plant health when in fact I've found plants flourish. So what I am still wanting to know is "how" adding higher amounts of ferts, like El or custom dosing PPS-Pro as I do, jeopardizes plant health.