If you have it, use it. There are better nutrients out there, but MG is a part of my nutrient mix.
I blend my own bloom nutrients to a ratio of 1-4-3. One part Nitrogen to four parts Phosphorous and three parts Potassium.
Adding a little 0-10-10(known around here as Master Bloom) would bring your blend much closer to optimum.
Use every source of nutrients, available. Organic sources are best, but are expensive to buy, and time consuming to gather(raking leaves, or driving to a volcano to mine nutrient rich clays, for instance).
Be practical, but read up on your plant's needs.