Miracle grow IS bad for the plants and the soil because it builds up salts in the soil, if you get lots of rain to wash out the soil, this is NOT much of a problem.
However, if you do not get lots of rain, it will render the soil useless over a period of time. There is some farmland in CA that is useless due to water soluable ferts like Miracle Grow.
Plus, mg does nothing to build the soil and make it healthier. I can understand using a little of it on potted plants but not in a garden. You could achieve the same results with some good garden teas I would think.
Miracle grow creates a high nitrogen soil, which will grow plants big, quick. This is not always a good thing for your plants and CAN make them susceptible to insects and other problems. Slow and steady wins the race in my opinion.
I reccomend soil building, adding compost, worms and other good soil building amendments to your dirt. This will give you the best garden and the healthiest soil. Ok, so what if you dont have the time or ability to do this for THIS years garden? Well using Miracle Grow for one year is NOT going to make the earth a barren place. IF you cannot get aroudn it, use it.