MG helps balance your calicium and alk. I fought many battles with slow coral growth and lack of colors due in part to MG. Not to mention my cal problems. If you use a quality salt you shouldn't have any problems with MG. I'm sure someone with more knowledge will chime in and add some more info, hope this helps!
An added benny to keeping your Mag level up is that it seems to erradicate bryopsis and GHA very quickly and efficiently from my experience. That and the ability to keep your calcium and alk at elevated levels without having precip events make it a must, and I have also noted better growth rates and polyp extension on my sps since getting it up to 1500ppm. Win-win if you ask me.
While I am skeptical about the bryopsis control angle .I favor high magnesium levels of at least 1285ppm which is the level noted for natural seawater.
Magnesium is taken up by corals in their skeletons. It also discourages excessive precipitation of calcium carbonate by taking the place of a calcium ion in a forming calcium carbonate crystal. In this way it makes it easier to maintain calcium and alkalinity .