In my opinion, probiotics are not a waste of money. We feed them to both our horses on a daily basis.....for different reasons.
We feed it to our 6 yr. Old OTTB because when we bought him his body score was a 2. Even with added weight supplements, he wouldn't gain weight...until we put him on SmartDigest from SmartPak.....now people keep mistaking him for an appendix!!!
he is kind of chubby, but really!!!
We also feed a multi vitamin supplement with probiotics added into it for our fat draft. She gets SmartPak s Easy Keeper Grass supplement because she gets NO grain and since we've moved here we have had a difficult time finding a hay dealer who sells QUALITY hay....oftentimes having dealers pass off hay that looks ten years old. So, since the quality of the hay varies from shipment to shipment, dealer to dealer and even bale to bale....we have Epona on a multi vitamin supplement.
IMO, probiotics are essential for hard keepers, older horses and horses with high metabolism like TBs or horses with digestive issues.