Well actually yes. The sun itself doesn't change spectrum's, but the distance from the earth, the elevation in the sky, as well as the atmosphere itself all play a part in the light spectrum that the reaches earth. This means that the time of year and the season affect the spectrum. During mid summer there's more blue spectrum of light (depending on your location on the planet). In the fall because the sun is lower in the sky, and the part the atmosphere plays in the light spectrum that reaches the earth, there's more of the red spectrum of light. The sun and the earth don't do this just for the sake of the plants, but plants have just evolved to adapt to their environment (like all life on the planet).
A myth is only a myth if you don't agree, or in the way you interpret the results. That is such a general statement, it's flawed from the beginning. Not all plants have the same requirements, also take either spectrum completely out of the picture (red or blue), and you likely wont have good results no mater what (depending on the plants your growing). Simply lumping all plants into one category is comparing grapes and oranges (flawed). Although from everything I have read, I don't believe that using just
hps during flowering would give the best results. If it were me and I had the money, I would use MH during vegetative growth, and both MH and HPS during flowering. Also from what I have read, the full spectrum
bulbs aren't what what the manufactures crack them up to be, as well as the combination MH and HPS bulbs (much like
led manufactures do). If I didn't have the money for both, I would go with MH. The plants will still flower, although yield and quality likely wont be the same (again depending on the specific plants your growing).
P.S.
I keep hearing about all this research, but have yet to see any of it. I can't speak for anyone else, but I don't trust any research I cant see. Otherwise you simply cant evaluate how the research was done, the objectiveness of the results and/or conclusions, nor the credibility of who conducted it. So just saying this is the results of research is meaningless to me (much like that job offer that says they'll call you, but never does). Simply doesn't help.