I use DFM (Direct Fed Microbial) by HorseTech for my minis every day. I feel it is an important preventative to help balance their systems from stress. They are in training and showing environments, this is just something that can help them. They are also on a daily dewormer, so I am of the opinion that this can help overcome any negative aspects of doing that.
I am very happy with the condition of all of my minis right now. They are on a timothy hay, plus whatever concentrate they are on (most are on Allegra Cadence, one is on a combo of beet pulp and LMF TACO instead), HorseTech Glanzen 3 for coat, skin and hooves, DFM, and daily dewormer. As we get closer to the show season, I will transition the rest onto beet pulp as well, it is so important to help carry water into their systems so they do not dehydrate under stress. I also wet their hay when we are traveling...anything to get water into them.