'In my opinion, this is a major flaw in WW.' (lack of attention to magnesium)
If WW recommends a daily multi-vitamin, that's as adventurous as they are going to get. They aren't going to go out on a limb and disagree with what is recommended for the RDA. As I said, those recommendations are changing for vitamin D (within the medical community) and so that has been discussed. But WW doesn't claim to be your cutting-edge source for new nutritional information. They would say, 'Discuss this with your doctor.'
It's a support group, basically, explaining calorie-reduction (don't eat more than your limit and exercise more). That all gets fit into a 45 minute meeting in which members' problems and successes are discussed. The topic of each week is the same across the U.S. (and perhaps other countries) and the group leaders go by the topic. This means that when you miss your usual meeting, you can go to any other meeting (no charge, that's included in your membership fee) and get the topic of that week. You can go to as many meetings as you want but you are only weighed once (that weight goes into a computer that puts it online for you to access when you sign in to the WW website).
So what you think WW should do- take a stance for more magnesium, for example - will happen when the medical community changes the RDA. Then that information will likely be spread through meetings the way the topic of Vitamin D spread through meetings - as the topic of the week.