from plants be every bit as good as the human form of vitamin D?
I believe the issue boils down to taking hormones from non-human sources. (Remember: Vitamin D is a hormone, a very powerful one at that.) Plants can be wonderful sources of flavonoids, fibers, protein, fats, vitamins, minerals, and other healthy components. But hormones?
There are other examples of non-human hormones being given to humans with undesirable or unpredictable effects:
--Xenoestrogens, phytoestrogens, and non-human mammalian estrogens--While non-human estrogens may partially mimic human estrogens, they can also block estrogen effects, or exert altogether novel effects. Non-human mammalian estrogens like Premarin can exert very peculiar (side-)effects, despite their role as prescription estrogen supplementation in humans.
--Progestins--The synthetic versions of human progesterone, like their non-human estrogen counterparts, exert weird effects that are a world apart from real progesterone.
--Sterols--Similar in structure to human cholesterol (while not a hormone, a building block for hormones), sterols have been used to reduce intestinal cholesterol absorption. However, if sterols are absorbed into the blood, they can enormously accelerate growth of atherosclerotic plaque.
--Anabolic steroids--These modifications of the testosterone molecule build muscle, but also cause liver cancer, kidney failure, violent behavior, suicide and homicidal behavior. That's not normal.
Outside of a pharmacologic effect (e.g., prednisone in place of human cortisol), there is no reason to take a non-human hormone in place of a human hormone. For that same reason, there is NO reason to take plant vitamin D2 (prescription or over-the-counter) in place of human vitamin D3.
If the non-human hormone is identical to the human form, then there is no difficulty. The best example of this are thyroid hormones from pigs. That's what Armour Thyroid is, a thyroid hormone replacement that works wonderfully well.
You will notice that virtually all of the examples of non-human hormones substituted for human hormones share one common motivation: profit. Synthetic or modified versions are more readily patent-protectable, unlike their natural counterparts which are not.
Vitamin D2 is an anemic facsimile of the real human hormone, vitamin D3 (cholecalciferol). Stay away from it.