Vitamin D is a hormone and it seems pretty clear that it plays a significant role in a number of body processes that contribute to overall health, so it doesn't seem likely that it is just a marker; it actually does help you directly, so it still makes sense to use supplements to raise your Vitamin D level if it's low.
What might well be true, IMO, is that if your Vitamin D level is low, and you can't get it back to normal with just sun exposure (which might well be an issue for people living at high latitudes, or as people age, since as I understand it your body's ability to synthesize Vitamin D from sun exposure decreases with age), you should not assume that supplements alone will be enough to protect your health. There might be other things you could do differently that would also help. In other words, low Vitamin D might be both a problem in itself (i.e., you need to get the level up) and an indirect indication of some other problem (which won't be solved just by getting the level up).
The problem is that, if the latter is the case, nobody seems to know very much about what other problems low Vitamin D might be an indication of, or what other interventions might be indicated if your Vitamin D is low, besides supplementation to get the level up. The only real advice that seems to be indicated, based on what we know now, is to take better care of your health in general--eat better, exercise more, get more sleep, etc.
What might well be true, IMO, is that if your Vitamin D level is low, and you can't get it back to normal with just sun exposure (which might well be an issue for people living at high latitudes, or as people age, since as I understand it your body's ability to synthesize Vitamin D from sun exposure decreases with age), you should not assume that supplements alone will be enough to protect your health. There might be other things you could do differently that would also help. In other words, low Vitamin D might be both a problem in itself (i.e., you need to get the level up) and an indirect indication of some other problem (which won't be solved just by getting the level up).
The problem is that, if the latter is the case, nobody seems to know very much about what other problems low Vitamin D might be an indication of, or what other interventions might be indicated if your Vitamin D is low, besides supplementation to get the level up. The only real advice that seems to be indicated, based on what we know now, is to take better care of your health in general--eat better, exercise more, get more sleep, etc.