Does it work? Well, according to the more positive studies, yes & no.
http://munin.uit.no/bitstream/handle/10037/3888/article.pdf?sequence=1
While it does improve the overall feed conversion ratio (FCR) the food used in that study mostly consisted of soybeans & wheat, and while it did have a positive affect on the intestinal microbiota at the initial stages of the feed trial, that apparently soon diminished over time. The longer it was fed, the lesser the positive affect. Also, while it was shown to elevate the expression of some immune related genes, it did not improve the disease resistance of koi challenged with Aeromonas hydrophila, which is one of the most problematic species of Aeromonas found in commercial aquaculture.
So what does that tell the average hobbyist that keeps ornamental species of fish?
Not a whole lot.
If you feed your fish a diet that is high in terrestrial based carbs such as gluten meal, corn, soybeans, wheat, potatoes, rice, etc - adding a probiotic such as Bacillus subtilis in large enough quantity could possibly increase the feed conversion ratio (depending on the species of fish), thereby causing an increase in growth, as well as a reduction in overall waste compared to feeding the same quality of feed, sans the probiotic. It could also help elevate the immune system of the fish, but so could numerous bioactive compounds found in natural raw ingredients such as, Antarctic Krill, Garlic, Spirulina, and micro algae such as Haematococcus pluvialis. Not to mention utilizing key vitamins & trace minerals at levels that far exceed the industry standards. As an example, instead of having a post extrusion level of 140 mg/kg of Vitamin C, having a level of 500+ mg/kg of Vitamin C. (with most of that coming from the raw ingredients themselves) Certainly some of these bacteria have been shown in some studies and using certain species of fish that they can improve the overall feed conversion ratio ....... but of foods that mostly consist of wheat & soybeans.
Of course that's a non issue for anyone that is NOT feeding large amounts of terrestrial based starch derived from corn, wheat, soybeans, potatos, etc to their fish.
Some of this is just plain old common sense, but some consumers can easily be sucked in by the hype.
Feed low cost crappy ingredients but add *special* bacteria to digest crappy ingredients, and charge twice the price of higher quality feeds that contain higher quality raw ingredients ....... simply because the former contain bacteria spores? Unfortunately many people buy into this type of hype or fad marketing.