What he said...
MiracleGro is alright for something like tomatoes...after all.. they are just tomatoes. the reason you don't want to use it for Marijuana is because it lacks specific nutrients that marijuana needs later in its life. it also contains to much of some nutrients. so although it may work at first, you'll end up needing to fertilize later in the plants life. this often results in over fertilization of certain nutrients.. and under fertilization of others...

In short, it's an inbalanced fertilizer for marijuana.
Idealy you should use a better grade soil, something with slow releasing nutrients...Scott's, Shultz.. there's alot of different names. usually you can use soils like that for two grow seasons.. i have

do what you can though.. if MG is what you want to use, nobody will stop you.